!177 fix：fopen入参校验不全导致访问异常

Merge pull request !177 from Zhaotianyu/1218fopen_fix

porting/liteos_m/kernel/src/stdio/fopen.c

@@ -12,7 +12,7 @@ FILE *fopen(const char *restrict filename, const char *restrict mode)
 	int flags;
 
 	/* Check for valid initial mode character */
-	if (!strchr("rwa", *mode)) {
+	if ((mode == NULL) || (!strchr("rwa", *mode))) {
 		errno = EINVAL;
 		return 0;
 	}