shared: fix typecheck in NM_PTRARRAY_LEN()

Previously, NM_PTRARRAY_LEN() would not work if the pointer type is
an opaque type, which is common. For example:

  NMConnection *const*connections = ...;

shared/nm-utils/nm-macros-internal.h

@@ -271,7 +271,8 @@ NM_G_ERROR_MSG (GError *error)
 		gsize _n = 0; \
 		\
 		if (_array) { \
-			_nm_unused typeof (*(_array[0])) *_array_check = _array[0]; \
+			_nm_unused gconstpointer _type_check_is_pointer = _array[0]; \
+			\
 			while (_array[_n]) \
 				_n++; \
 		} \