perf parse-events: Check if the software events array slots are populated

To avoid a NULL pointer dereference when the kernel supports the new
feature but the tooling still hasn't an entry for it.

This happened with the recently added PERF_COUNT_SW_CGROUP_SWITCHES
software event.
Reported-by: Thomas Richter <tmricht@linux.ibm.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Sumanth Korikkar <sumanthk@linux.ibm.com>
Link: https://lore.kernel.org/linux-perf-users/YKVESEKRjKtILhog@kernel.org/Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/parse-events.c

@@ -2932,9 +2932,14 @@ void print_symbol_events(const char *event_glob, unsigned type,
 	}
 
 	for (i = 0; i < max; i++, syms++) {
+		/*
+		 * New attr.config still not supported here, the latest
+		 * example was PERF_COUNT_SW_CGROUP_SWITCHES
+		 */
+		if (syms->symbol == NULL)
+			continue;
 
-		if (event_glob != NULL && syms->symbol != NULL &&
-		    !(strglobmatch(syms->symbol, event_glob) ||
+		if (event_glob != NULL && !(strglobmatch(syms->symbol, event_glob) ||
 		      (syms->alias && strglobmatch(syms->alias, event_glob))))
 			continue;