env: Don't match empty variable name in env_match()

Do we really allow zero-length variable name? I guess not.

Signed-off-by: Marek BehĂșn <marek.behun@nic.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/nvedit.c b/cmd/nvedit.c
index 5b1d4c2..8d53579 100644
--- a/cmd/nvedit.c
+++ b/cmd/nvedit.c
@@ -708,7 +708,7 @@
 
 static const char *env_match(const char *p, const char *s1)
 {
-	if (s1 == NULL)
+	if (s1 == NULL || *s1 == '\0')
 		return NULL;
 
 	while (*s1 == *p++)