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.comSigned-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240621170528.608772-4-namhyung@kernel.org

tools/perf/util/dsos.c

@@ -164,6 +164,9 @@ static struct dso *__dsos__find_by_longname_id(struct dsos *dsos,
 	};
 	struct dso **res;
 
+	if (dsos->dsos == NULL)
+		return NULL;
+
 	if (!dsos->sorted) {
 		if (!write_locked) {
 			struct dso *dso;