env: Check for NULL pointer in envmatch()

If the pointer passed into envmatch() is NULL, return -1 instead of
crashing.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>

common/cmd_nvedit.c

@@ -650,6 +650,9 @@ U_BOOT_CMD(
  */
 int envmatch(uchar *s1, int i2)
 {
+	if (s1 == NULL)
+		return -1;
+
 	while (*s1 == env_get_char(i2++))
 		if (*s1++ == '=')
 			return i2;

tools/env/fw_env.c

@@ -1057,6 +1057,8 @@ exit:
 
 static char *envmatch (char * s1, char * s2)
 {
+	if (s1 == NULL || s2 == NULL)
+		return NULL;
 
 	while (*s1 == *s2++)
 		if (*s1++ == '=')