libkvs/watch: match va_start/va_end on error

Fix cppcheck error:

[src/common/libkvs/kvs_watch.c:109]: (error) Memory leak: ctx
[src/common/libkvs/kvs_watch.c:363]: (error) va_list 'ap' was opened but
not closed by va_end().
[src/common/libkvs/kvs_watch.c:429]: (error) va_list 'ap' was opened but
not closed by va_end().

src/common/libkvs/kvs_watch.c

@@ -358,11 +358,12 @@ int kvs_watch_once_dir (flux_t *h, kvsdir_t **dirp, const char *fmt, ...)
     int rc;
 
     va_start (ap, fmt);
-    if (vasprintf (&key, fmt, ap) < 0) {
+    rc = vasprintf (&key, fmt, ap);
+    va_end (ap);
+    if (rc < 0) {
         errno = ENOMEM;
         return -1;
     }
-    va_end (ap);
     rc = watch_once_dir (h, key, dirp);
     free (key);
     return rc;
@@ -424,11 +425,12 @@ int kvs_watch_dir (flux_t *h, kvs_set_dir_f set, void *arg,
     int rc;
 
     va_start (ap, fmt);
-    if (vasprintf (&key, fmt, ap) < 0) {
+    rc = vasprintf (&key, fmt, ap);
+    va_end (ap);
+    if (rc < ) {
         errno = ENOMEM;
         return -1;
     }
-    va_end (ap);
     rc = watch_dir (h, key, set, arg);
     free (key);
     return rc;