Fix low-probability memory leak in regex execution.

After an internal failure in shortest() or longest() while pinning down the
exact location of a match, find() forgot to free the DFA structure before
returning.  This is pretty unlikely to occur, since we just successfully
ran the "search" variant of the DFA; but it could happen, and it would
result in a session-lifespan memory leak since this code uses malloc()
directly.  Problem seems to have been aboriginal in Spencer's library,
so back-patch all the way.

In passing, correct a thinko in a comment I added awhile back about the
meaning of the "ntree" field.

I happened across these issues while comparing our code to Tcl's version
of the library.

src/backend/regex/regexec.c

@@ -323,7 +323,11 @@ find(struct vars * v,
 						   (chr **) NULL, &hitend);
 		else
 			end = longest(v, d, begin, v->stop, &hitend);
-		NOERR();
+		if (ISERR())
+		{
+			freedfa(d);
+			return v->err;
+		}
 		if (hitend && cold == NULL)
 			cold = begin;
 		if (end != NULL)