Avoid possible deference NULL pointer (src/backend/optimizer/path/allpaths.c)

Hi.

Per Coverity.

All call sites of function *get_cheapest_path_for_pathkeys* checks
for NULL returns.

So, it is highly likely that the function will return NULL.

IMO, the Assert in this particular call, is not fully effective.

Fix removing the Assert and always check if the return is NULL.

best regards,
Ranier Vilela