env: fix crash using default -f -a

env default -a -f calls env_check_apply on all existing environment
variables with a NULL value for "newval" as a way of cleaning up.
This causes string manipulation functions to crash on most architectures.
So replace a NULL argument with an empty string.
Reported-By: NStefano Babic <sbabic@denx.de>
Signed-off-by: NGerlando Falauto <gerlando.falauto@keymile.com>
Tested-by: NStefano Babic <sbabic@denx.de>

common/cmd_nvedit.c

@@ -215,6 +215,9 @@ int env_check_apply(const char *name, const char *oldval,
 {
 	int   console = -1;
 
+	/* Default value for NULL to protect string-manipulating functions */
+	newval = newval ? : "";
+
 	/* Check for console redirection */
 	if (strcmp(name, "stdin") == 0)
 		console = stdin;