Further tweak memory management for regex DFAs.

 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.