Avoid crash in resolve_dynamic_struct

Avoid crash in resolve_dynamic_struct

resolve_dynamic_struct says:

  gdb_assert (type->num_fields () > 0);

However, a certain Ada program has a structure with no fields but with
a dynamic size, causing this assertion to fire.

It is difficult to be certain, but we think this is a compiler bug.
However, in the meantime this assertion does not seem to be checking
any kind of internal consistency; so this patch removes it.

gdb/ChangeLog
2021-02-09  Tom Tromey  <tromey@adacore.com>

	* gdbtypes.c (resolve_dynamic_struct): Handle structure with no
	fields.