rtspd: fix to check realloc() result

[vszakats]

Also enable bugprone-suspicious-realloc-usage clang-tidy option
to verify.

Fixing:

tests/server/rtspd.c:328:37: error: 'req->rtp_buffer' may be set to null if 'realloc' fails,
 which may result in a leak of the original buffer
 [bugprone-suspicious-realloc-usage,-warnings-as-errors]
  328 |                   req->rtp_buffer = realloc(req->rtp_buffer,
      |                   ~~~~~~~~~~~~~~~   ^       ~~~~~~~~~~~~~~~

Ref: https://clang.llvm.org/extra/clang-tidy/checks/bugprone/suspicious-realloc-usage.html