[gdb] Fix prop->const_val uses in gdbtypes.c

[gdb] Fix prop->const_val uses in gdbtypes.c

After commit 66d6346b25 "gdb: remove TYPE_DYN_PROP_ADDR", I run into:
...
FAIL: gdb.fortran/class-allocatable-array.exp: print this%_data%b
...
(and 185 more FAILs, all for fortran test-cases).

The commit replaces "!x" by "x != 0".

Fix this by using "x == 0" instead.

Build and tested on x86_64-linux.

gdb/ChangeLog:

2020-08-05  Tom de Vries  <tdevries@suse.de>

	* gdbtypes.c (type_not_allocated, type_not_associated): Use
	"prop->const_val () == 0" instead of "prop->const_val () != 0".