Further tweak memory management for regex DFAs.

Coverity is still unhappy after commit 190c79884, and after looking
closer I think it might be onto something.  The callers of newdfa()
typically drop out if v->err has been set nonzero, which newdfa()
is faithfully doing if it fails.  However, what if v->err was already
nonzero before we entered newdfa()?  Then newdfa() could succeed and
the caller would promptly leak its result.

I don't think this scenario can actually happen, but the predicate
"v->err is always zero when newdfa() is called" seems difficult to be
entirely sure of; there's a good deal of code that potentially could
get that wrong.

It seems better to adjust the callers to directly check for a null
result instead of relying on ISERR() tests.  This is slightly cheaper
than the previous coding anyway.

Lacking evidence that there's any real bug, no back-patch.

src/backend/regex/rege_dfa.c

@@ -604,6 +604,8 @@ lastcold(struct vars *v,
 
 /*
  * newdfa - set up a fresh DFA
+ *
+ * Returns NULL (and sets v->err) on failure.
  */
 static struct dfa *
 newdfa(struct vars *v,