spa: support: log-impl: check the return value of `strrchr()`

Previously, the return value of `strrchr()` was not checked
in `spa_log_impl_logv()` which could cause a segmentation fault
in `snprintf()` if the `file` string argument does not contain
any directory separators ('/').

For example,

  ./build/spa/tools/spa-inspect ./build/spa/plugins/alsa/libspa-alsa.so

could run into this problem:

  Program received signal SIGSEGV, Segmentation fault.
  0x00007ffff7f1d505 in __strlen_avx2 () from /usr/lib/libc.so.6
  (gdb) bt
  #0  0x00007ffff7f1d505 in __strlen_avx2 () from /usr/lib/libc.so.6
  #1  0x00007ffff7e29408 in __vfprintf_internal () from /usr/lib/libc.so.6
  #2  0x00007ffff7e3a19a in __vsnprintf_internal () from /usr/lib/libc.so.6
  #3  0x00007ffff7e146f6 in snprintf () from /usr/lib/libc.so.6
  #4  0x0000555555558818 in spa_log_impl_logv (object=<optimized out>, level=SPA_LOG_LEVEL_INFO,
      file=0x7fffffffd4d0 "[I][", line=260, func=0x7ffff7ce8090 "error_node", fmt=<optimized out>,
      args=0x7fffffffd920) at ../spa/include/spa/support/log-impl.h:49
  #5  0x00007ffff7d8c69e in alsa_error_handler (file=<optimized out>, line=<optimized out>,
      function=<optimized out>, err=<optimized out>, fmt=<optimized out>)
      at ../spa/plugins/alsa/acp/alsa-util.c:866
  #6  0x00007ffff7cd6a8f in ?? () from /usr/lib/libasound.so.2
  #7  0x00007ffff7cdb55e in snd_use_case_mgr_open () from /usr/lib/libasound.so.2
  #8  0x00007ffff7d8940f in pa_alsa_ucm_query_profiles (ucm=ucm@entry=0x55555556ba28, card_index=0)
      at ../spa/plugins/alsa/acp/alsa-ucm.c:752
  #9  0x00007ffff7d6e3c2 in acp_card_new (index=0, props=props@entry=0x7fffffffdc50)
      at ../spa/plugins/alsa/acp/acp.c:1508
  #10 0x00007ffff7d29b7a in impl_init (factory=<optimized out>, handle=0x55555556b540,
      info=<optimized out>, support=<optimized out>, n_support=<optimized out>)
      at ../spa/plugins/alsa/alsa-acp-device.c:963
  #11 0x0000555555558429 in inspect_factory (factory=0x7ffff7daefa0 <spa_alsa_acp_device_factory>,
      data=0x7fffffffdcf0) at ../spa/tools/spa-inspect.c:231
  #12 main (argc=<optimized out>, argv=<optimized out>) at ../spa/tools/spa-inspect.c:309

as in that particular case, the filename was returned by libasound,
and it was just "parser.c".

Furthermore, separate the static variable from the rest, and apply
the `const` qualifier to the pointers in the `levels` array.