perf tools: Free temporary 'sys' string in read_event_files()

For each system in a given pevent, read_event_files() reads in a
temporary 'sys' string.  Be sure to free this string before moving onto
to the next system and/or leaving read_event_files().

Fixes the following coverity complaints:

  Error: RESOURCE_LEAK (CWE-772):

  tools/perf/util/trace-event-read.c:343: overwrite_var: Overwriting
  "sys" in "sys = read_string()" leaks the storage that "sys" points to.

  tools/perf/util/trace-event-read.c:353: leaked_storage: Variable "sys"
  going out of scope leaks the storage it points to.
Signed-off-by: Sanskriti Sharma <sansharm@redhat.com>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Cc: Joe Lawrence <joe.lawrence@redhat.com>
Link: http://lkml.kernel.org/r/1538490554-8161-6-git-send-email-sansharm@redhat.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/trace-event-read.c

@@ -347,10 +347,13 @@ static int read_event_files(struct tep_handle *pevent)
 		for (x=0; x < count; x++) {
 			size = read8(pevent);
 			ret = read_event_file(pevent, sys, size);
-			if (ret)
+			if (ret) {
+				free(sys);
 				return ret;
 			}
 		}
+		free(sys);
+	}
 	return 0;
 }