fixed segmentation fault in status command with http stream

Fixes https://github.com/cmus/cmus/issues/73


Steps to reproduce:

    cmus-remote ‘http://rpod.ru/get/303797/271934/download/Boris%20Grebenshchikov%20-%20Aerostat%20Radio%20vol.420.mp3'
    cmus-remote --play
    cmus-remote --query

Version:

    cmus v2.5.0-93-gb47e190

reproduce on linux and mac os

GDB:

    Linux:
    Program received signal SIGSEGV, Segmentation fault.
    0xb6dfa834 in strlen () from /lib/arm-linux-gnueabihf/libc.so.6

    (gdb) where
    #0  0xb6dfa834 in strlen () from /lib/arm-linux-gnueabihf/libc.so.6
    #1  0x00026458 in escape (str=0x0) at misc.c:94
    #2  0x0002dbcc in cmd_status (client=0x97b70) at server.c:111
    #3  read_commands (client=0x97b70) at server.c:242
    #4  0x0002dd84 in server_serve (client=<optimized out>) at server.c:295
    #5  0x00036d88 in main_loop () at ui_curses.c:2183
    #6  0x00011a44 in main (argc=<optimized out>, argv=0x0) at ui_curses.c:2481

    ---
    Mac OS:

    Reading symbols for shared libraries .
    Program received signal EXC_BAD_ACCESS, Could not access memory.
    Reason: KERN_INVALID_ADDRESS at address: 0x0000000000000000
    0x00007fff918c1812 in strlen ()
    (gdb) where
    #0  0x00007fff918c1812 in strlen ()
    #1  0x000000010001819f in escape (str=0x0) at misc.c:93
    #2  0x0000000100020b3d in read_commands [inlined] () at /Users/msa/var/src/cmus/server.c:114
    #3  0x0000000100020b3d in server_serve (client=<value temporarily unavailable, due to optimizations>) at server.c:295
    #4  0x0000000100029c7f in main (argc=0, argv=0x7fff5fbff968) at ui_curses.c:2158