MIPS: fw: Allow firmware to pass a empty env

fw_getenv will use env entry to determine style of env,
however it is legal for firmware to just pass a empty list.

Check if first entry exist before running strchr to avoid
null pointer dereference.

Cc: [e-mail]
Link: https://github.com/clbr/n64bootloader/issues/5
Signed-off-by: Jiaxun Yang <[e-mail]>
Signed-off-by: Thomas Bogendoerfer <[e-mail]>

arch/mips/fw/lib/cmdline.c

···
 {
 	char *result = NULL;
 
-	if (_fw_envp != NULL) {
+	if (_fw_envp != NULL && fw_envp(0) != NULL) {
 		/*
 		 * Return a pointer to the given environment variable.
 		 * YAMON uses "name", "value" pairs, while U-Boot uses