libbsc/system_information.c: Fix off-by-one error in si4 generation with CBCH enabled

==25637==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7fff3f587c43 at pc 0x5bf591 bp 0x7fff3f587630 sp 0x7fff3f587628
READ of size 4 at 0x7fff3f587c43 thread T0
    #0 0x5bf590 in tv_fixed_put /home/alphaone/local/osmo-asan/include/osmocom/gsm/tlv.h:237
    #1 0x5b7e14 in generate_si4 /home/alphaone/scm/osmo/openbsc/openbsc/src/libbsc/system_information.c:607
    #2 0x5b488b in gsm_generate_si /home/alphaone/scm/osmo/openbsc/openbsc/src/libbsc/system_information.c:882
    #3 0x4cb247 in gsm_bts_trx_set_system_infos /home/alphaone/scm/osmo/openbsc/openbsc/src/libbsc/bsc_init.c:166
    #4 0x4d3c26 in bootstrap_rsl /home/alphaone/scm/osmo/openbsc/openbsc/src/libbsc/bsc_init.c:272
    #5 0x4ced44 in inp_sig_cb /home/alphaone/scm/osmo/openbsc/openbsc/src/libbsc/bsc_init.c:316
    #6 0x7f4f15b563d7 in osmo_signal_dispatch /home/alphaone/scm/osmo/libosmocore/src/signal.c:105
    #7 0x7f4f156c0e3f in e1inp_int_snd_event (/home/alphaone/local/osmo-asan/lib/libosmoabis.so.4+0x17e3f)
    #8 0x7f4f156be7e5 in e1inp_event (/home/alphaone/local/osmo-asan/lib/libosmoabis.so.4+0x157e5)
    #9 0x583a6a in ipaccess_sign_link /home/alphaone/scm/osmo/openbsc/openbsc/src/libbsc/bts_ipaccess_nanobts.c:675
    #10 0x7f4f156e63b0 in handle_ts1_read (/home/alphaone/local/osmo-asan/lib/libosmoabis.so.4+0x3d3b0)
    #11 0x7f4f156e4f4e in ipaccess_fd_cb (/home/alphaone/local/osmo-asan/lib/libosmoabis.so.4+0x3bf4e)
    #12 0x7f4f15b540b6 in osmo_select_main /home/alphaone/scm/osmo/libosmocore/src/select.c:160
    #13 0x43c656 in main /home/alphaone/scm/osmo/openbsc/openbsc/src/osmo-nitb/bsc_hack.c:355
    #14 0x7f4f1462e4bc (/lib64/libc.so.6+0x224bc)
    #15 0x43b6cc (/home/alphaone/local/osmo-asan/bin/osmo-nitb+0x43b6cc)
Address 0x7fff3f587c43 is located in stack of thread T0 at offset 483 in frame
    #0 0x5b712f in generate_si4 /home/alphaone/scm/osmo/openbsc/openbsc/src/libbsc/system_information.c:580
  This frame has 8 object(s):
    [32, 40) ''
    [96, 104) ''
    [160, 164) 'rc'
    [224, 232) 'si4'
    [288, 296) 'cbch_lchan'
    [352, 360) 'restoct'
    [416, 420) 'l2_plen'
    [480, 483) 'cd'
HINT: this may be a false positive if your program uses some custom stack unwind mechanism or swapcontext
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-buffer-overflow /home/alphaone/local/osmo-asan/include/osmocom/gsm/tlv.h:237 tv_fixed_put
libbsc/chan_alloc: Fix size of pchan to hold the +CBCH channels as well

show net with an CCCH+SDCCH/4+CBCH channel active caused bts_chan_load to read
from invalid memory. Fix this by making sure the pchan array is large enough.

==30346==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7fff9bdc5dc8 at pc 0x5aeece bp 0x7fff9bdc5350 sp 0x7fff9bdc5348
READ of size 4 at 0x7fff9bdc5dc8 thread T0
    #0 0x5aeecd in bts_chan_load /home/alphaone/scm/osmo/openbsc/openbsc/src/libbsc/chan_alloc.c:490
    #1 0x5af706 in network_chan_load /home/alphaone/scm/osmo/openbsc/openbsc/src/libbsc/chan_alloc.c:511
    #2 0x4b7410 in net_dump_vty /home/alphaone/scm/osmo/openbsc/openbsc/src/libbsc/bsc_vty.c:208
    #3 0x4b5f23 in show_net /home/alphaone/scm/osmo/openbsc/openbsc/src/libbsc/bsc_vty.c:227
    #4 0x7fdabaa425bd in cmd_execute_command_real /home/alphaone/scm/osmo/libosmocore/src/vty/command.c:2042
    #5 0x7fdabaa3f124 in cmd_execute_command /home/alphaone/scm/osmo/libosmocore/src/vty/command.c:2077
    #6 0x7fdabaa850e9 in vty_command /home/alphaone/scm/osmo/libosmocore/src/vty/vty.c:402
    #7 0x7fdabaa75962 in vty_execute /home/alphaone/scm/osmo/libosmocore/src/vty/vty.c:666
    #8 0x7fdabaa6d947 in vty_read /home/alphaone/scm/osmo/libosmocore/src/vty/vty.c:1408
    #9 0x7fdabaa9165f in client_data /home/alphaone/scm/osmo/libosmocore/src/vty/telnet_interface.c:119
    #10 0x7fdaba7860b6 in osmo_select_main /home/alphaone/scm/osmo/libosmocore/src/select.c:160
    #11 0x43c656 in main /home/alphaone/scm/osmo/openbsc/openbsc/src/osmo-nitb/bsc_hack.c:355
    #12 0x7fdab92604bc (/lib64/libc.so.6+0x224bc)
    #13 0x43b6cc (/home/alphaone/local/osmo-asan/bin/osmo-nitb+0x43b6cc)
Address 0x7fff9bdc5dc8 is located in stack of thread T0 at offset 232 in frame
    #0 0x4b5faf in net_dump_vty /home/alphaone/scm/osmo/openbsc/openbsc/src/libbsc/bsc_vty.c:182
  This frame has 3 object(s):
    [32, 40) ''
    [96, 104) ''
    [160, 224) 'pl'
HINT: this may be a false positive if your program uses some custom stack unwind mechanism or swapcontext
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-buffer-overflow /home/alphaone/scm/osmo/openbsc/openbsc/src/libbsc/chan_alloc.c:490 bts_chan_load