Fix poorly thought-through code from commit 5c3c3cd0a3046339.

Fix poorly thought-through code from commit 5c3c3cd0a3046339.

It's not entirely clear to me whether PyString_AsString can return
null (looks like the answer might vary between Python 2 and 3).
But in any case, this code's attempt to cope with the possibility
was quite broken, because pstrdup() neither allows a null argument
nor ever returns a null.

Moreover, the code below this point assumes that "message" is a
palloc'd string, which would not be the case for a dgettext result.

Fix both problems by doing the pstrdup step separately.