c: Fix up error-recovery on functions initialized as variables [PR109412]

The change to allow empty initializers in C broke error-recovery on the
following testcase.  We are emitting function %qD is initialized like a
variable error early; if the initializer is non-empty, we just emit
another error that the initializer is invalid.  Previously if it was empty,
we'd emit another error that scalar is being initialized by empty
initializer (not really correct), but now we instead just try to
build_zero_cst for the FUNCTION_TYPE and ICE on it.

The following patch just emits the same diagnostics for the empty
initializers as we emit for the non-empty ones.

2023-04-27  Jakub Jelinek  <jakub@redhat.com>

	PR c/107682
	PR c/109412
	* c-typeck.cc (pop_init_level): If constructor_type is FUNCTION_TYPE,
	reject empty initializer as invalid.

	* gcc.dg/pr109412.c: New test.