use_count: Return if uc is NULL

The documentation for osmo_use_count_get_put states the return value is
"Negative on range violations or USE_LIST == NULL, the use_cb()'s
return value, or 0 on success"

However, the code in _osmo_use_count_get_put doesn't check if uc is NULL
- instead it would crash in osmo_use_count_find() where it is dereferenced.
Add a check for uc and return -EINVAL if it is NULL.

Change-Id: I792563696860a3100e95cafdd5fe57511819ef56
Related: SYS#5895

src/use_count.c

@@ -235,6 +235,8 @@ int _osmo_use_count_get_put(struct osmo_use_count *uc, const char *use, int32_t
 {
 	struct osmo_use_count_entry *e;
 	int32_t old_use_count;
+	if (!uc)
+		return -EINVAL;
 	if (!change)
 		return 0;