gdb: Fix sizeof for dynamic types other than arrays

gdb: Fix sizeof for dynamic types other than arrays

In commit:

   commit 37cc0caeca4c9a8552370040f4cfeaeceaa03369
   Date:   Wed Jul 18 13:38:35 2018 +0200
   [gdb/exp] Interpret size of vla with unknown size as <optimized out>

All dynamic types are treated as arrays in the 'sizeof' code path,
which means that structures can incorrectly be treated as arrays.
This can cause a failure in the gdb.base/vla-datatypes.exp test
script.

This commit adds a check that we do have an array before checking the
array bounds, and I also check that the array index type is dynamic
too.  This second check probably isn't strictly necessary, but
shouldn't hurt, a non-dynamic index type shouldn't have undefined high
bound.

gdb/ChangeLog:

	* eval.c (evaluate_subexp_for_sizeof): Check for array type before
	checking array bounds are defined.