perf machine: Null-terminate version char array upon fgets(/proc/version) error

If fgets() fails due to any other error besides end-of-file, the version
char array may not even be null-terminated.

Signed-off-by: Donald Yandt <donald.yandt@gmail.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Avi Kivity <avi@scylladb.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Yanmin Zhang <yanmin_zhang@linux.intel.com>
Fixes: a1645ce12a ("perf: 'perf kvm' tool for monitoring guest performance from host")
Link: http://lkml.kernel.org/r/20190514110100.22019-1-donald.yandt@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/machine.c

@@ -1234,8 +1234,9 @@ static char *get_kernel_version(const char *root_dir)
 	if (!file)
 		return NULL;
 
-	version[0] = '\0';
 	tmp = fgets(version, sizeof(version), file);
+	if (!tmp)
+		*version = '\0';
 	fclose(file);
 
 	name = strstr(version, prefix);