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 ru

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: stable@vger.kernel.org
Link: https://github.com/clbr/n64bootloader/issues/5
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

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