perf tools: Fix a compiler warning of NULL pointer

perf tools: Fix a compiler warning of NULL pointer

A compiler warning on the second argument of bsearch() should not be
NULL, but there's a case we might pass it.  Let's return early if we
don't have any DSOs to search in __dsos__find_by_longname_id().

  util/dsos.c:184:8: runtime error: null pointer passed as argument 2, which is declared to never be null

Reported-by: kernel test robot <oliver.sang@intel.com>
Reviewed-by: Kan Liang <kan.liang@linux.intel.com>
Closes: https://lore.kernel.org/oe-lkp/202406180932.84be448c-oliver.sang@intel.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240621170528.608772-4-namhyung@kernel.org