tracing: Reset parser->buffer to allow multiple "puts"

trace_parser_put() simply frees the allocated parser buffer. But it does not
reset the pointer that was freed. This means that if trace_parser_put() is
called on the same parser more than once, it will corrupt the allocation
system. Setting parser->buffer to NULL after free allows it to be called
more than once without any ill effect.
Acked-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace.c

@@ -1193,6 +1193,7 @@ int trace_parser_get_init(struct trace_parser *parser, int size)
 void trace_parser_put(struct trace_parser *parser)
 {
 	kfree(parser->buffer);
+	parser->buffer = NULL;
 }
 
 /*