Fix assertion failure in coerce_unspec_val_to_type

Fix assertion failure in coerce_unspec_val_to_type

coerce_unspec_val_to_type does:

      set_value_address (result, value_address (val));

However, this is only valid for lval_memory.  This patch changes this
code to only set the address for lval_memory values.

This seems like an ordinary oversight in coerce_unspec_val_to_type,
and a test case would be difficult to write, so I'm submitting it
without a test case.

Tested on x86-64 Fedora 29; plus using an Ada program that exhibits
the bug (but which cannot be shared).

gdb/ChangeLog
2019-05-14  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (coerce_unspec_val_to_type): Only set address when
	value is not lval_memory.