mrdb: fix two memory leaks

```
==17179==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 20 byte(s) in 4 object(s) allocated from:
    #0 0x4a005e in realloc (/[REDACTED]/mruby-git/bin/mrdb+0x4a005e)
    #1 0x4c6dcd in mrb_realloc_simple /[REDACTED]/mruby-git/src/gc.c:177:8
    #2 0x4c7094 in mrb_realloc /[REDACTED]/mruby-git/src/gc.c:192:8
    #3 0x4bdbc7 in dirname /[REDACTED]/mruby-git/mrbgems/mruby-bin-debugger/tools/mrdb/apilist.c:76:14
    #4 0x4bda30 in mrb_debug_get_source /[REDACTED]/mruby-git/mrbgems/mruby-bin-debugger/tools/mrdb/apilist.c:180:20
    #5 0x4c3aca in dbgcmd_list /[REDACTED]/mruby-git/mrbgems/mruby-bin-debugger/tools/mrdb/cmdmisc.c:410:21
    #6 0x4c1ceb in mrb_debug_break_hook /[REDACTED]/mruby-git/mrbgems/mruby-bin-debugger/tools/mrdb/mrdb.c:634:10
    #7 0x4c1b36 in mrb_code_fetch_hook /[REDACTED]/mruby-git/mrbgems/mruby-bin-debugger/tools/mrdb/mrdb.c:616:3
    #8 0x4d960e in mrb_context_run /[REDACTED]/mruby-git/src/vm.c:1588:7
    #9 0x4dcbf1 in mrb_toplevel_run_keep /[REDACTED]/mruby-git/src/vm.c:2391:12
    #10 0x542730 in load_exec /[REDACTED]/mruby-git/src/parse.y:5599:7
    #11 0x4c0ab5 in main /[REDACTED]/mruby-git/mrbgems/mruby-bin-debugger/tools/mrdb/mrdb.c:692:9
    #12 0x7f1feb25303f in __libc_start_main (/usr/lib/libc.so.6+0x2003f)

SUMMARY: AddressSanitizer: 20 byte(s) leaked in 4 allocation(s).
```
```
==18347==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 105 byte(s) in 3 object(s) allocated from:
    #0 0x4a005e in realloc (/[REDACTED]/mruby-git/bin/mrdb+0x4a005e)
    #1 0x4c6e0d in mrb_realloc_simple /[REDACTED]/mruby-git/src/gc.c:177:8
    #2 0x4c70d4 in mrb_realloc /[REDACTED]/mruby-git/src/gc.c:192:8
    #3 0x4be6b3 in mrb_debug_set_break_line /[REDACTED]/mruby-git/mrbgems/mruby-bin-debugger/tools/mrdb/apibreak.c:205:14
    #4 0x4c5b9d in dbgcmd_break /[REDACTED]/mruby-git/mrbgems/mruby-bin-debugger/tools/mrdb/cmdbreak.c:331:13
    #5 0x4c1d2b in mrb_debug_break_hook /[REDACTED]/mruby-git/mrbgems/mruby-bin-debugger/tools/mrdb/mrdb.c:634:10
    #6 0x4c1b76 in mrb_code_fetch_hook /[REDACTED]/mruby-git/mrbgems/mruby-bin-debugger/tools/mrdb/mrdb.c:616:3
    #7 0x4d964e in mrb_context_run /[REDACTED]/mruby-git/src/vm.c:1588:7
    #8 0x4dcc31 in mrb_toplevel_run_keep /[REDACTED]/mruby-git/src/vm.c:2391:12
    #9 0x4f37f4 in mrb_load_irep_file_cxt /[REDACTED]/mruby-git/src/load.c:705:9
    #10 0x4c0a88 in main /[REDACTED]/mruby-git/mrbgems/mruby-bin-debugger/tools/mrdb/mrdb.c:687:9
    #11 0x7f31aac5d03f in __libc_start_main (/usr/lib/libc.so.6+0x2003f)

SUMMARY: AddressSanitizer: 105 byte(s) leaked in 3 allocation(s).
```