Add two missing return values in gdb.python/py-nested-maps.c

Add two missing return values in gdb.python/py-nested-maps.c

Two functions in gdb.python/py-nested-maps.c are missing return
values.  This causes clang to fail to compile the file with the
following error:
  warning: control reaches end of non-void function [-Wreturn-type]

This commit fixes, by causing the two functions to return pointers
to the objects they've just allocated and initialized.  I didn't
investigate how this test had been passing with other compilers;
I'm assuming serendipity, that in each function the value to be
returned was already in the register it would need to be in to be
the function's return value.

gdb/testsuite/ChangeLog:

	* gdb.python/py-nested-maps.c (create_map): Add missing return
	value.
	(create_map_map): Likewise.