c: Fix up error-recovery on non-empty VLA initializers [PR109409]

On the following testcase we ICE, because after we emit the
variable-sized object may not be initialized except with an empty initializer
error we don't really reset the initializer to error_mark_node and then at
-Wformat checking time we ICE on seeing STRING_CST initializer for a VLA.

The following patch just arranges for error_mark_node to be returned after
the error diagnostics.

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

	PR c/109409
	* c-parser.cc (c_parser_initializer): Move diagnostics about
	initialization of variable sized object with non-empty initializer
	after c_parser_expr_no_commas call and ret.set_error (); after it.

	* gcc.dg/pr109409.c: New test.