Avoid crash with GCC trunk

With GCC trunk, gdb.ada/access_to_packed_array.exp causes a GDB crash.
The problem is that ptype tries to resolve a dynamic type.  However,
the inferior is not running, so there are no frames.

This patch updates dwarf2_evaluate_loc_desc::get_frame_base to handle
this situation.

gdb/ChangeLog
2021-05-17  Tom Tromey  <tromey@adacore.com>

	* dwarf2/loc.c (dwarf2_evaluate_loc_desc::get_frame_base): Throw
	if frame is null.