scripts/tracepoint-update: Fix memory leak in add_string() on failure

When realloc() fails in add_string(), the function returns -1 but leaves
*vals pointing to the previously allocated memory. This can cause memory
leaks in callers like make_trace_array() that return on error without
freeing the partially built array.

Fix this by freeing *vals and setting it to NULL when realloc() fails.
This makes the error handling self-contained in add_string() so callers
don't need to handle cleanup on failure.

This bug is found by my static analysis tool and my code review.

Cc: Masami Hiramatsu <[e-mail]>
Cc: Mathieu Desnoyers <[e-mail]>
Fixes: e30f8e61e2518 ("tracing: Add a tracepoint verification check at build time")
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Weigang He <[e-mail]>
Signed-off-by: Steven Rostedt (Google) <[e-mail]>

scripts/tracepoint-update.c

···
 		array = realloc(array, sizeof(char *) * size);
 		if (!array) {
 			fprintf(stderr, "Failed memory allocation\n");
+			free(*vals);
+			*vals = NULL;
 			return -1;
 		}
 		*vals = array;