mips: Fix an ICE caused by r12-7962

mips: Fix an ICE caused by r12-7962

DECL_SIZE(x) is NULL if x is a flexible array member, but I forgot to
check it in r12-7962.  Then if we increase the size of a struct with
flexible array member (by using aligned attribute), the code will
dereference NULL trying to use the "size" of the flexible array member.

gcc/

	* config/mips/mips.cc (mips_function_arg): Check if DECL_SIZE is
	NULL before dereferencing it.

gcc/testsuite/

	* gcc.target/mips/pr102024-4.c: New test.