theme room vampire fix

From a reddit thread:  a 'mausoleum' theme room picked a vampire for
its occupant and applied the wait-for-you strategy to it.  Hero's ESP
or monster detection showed a meditating vampire bat.  Change monster
creation by the special level loader (which also handles theme rooms)
to force such a creature into its normal vampire form.

That revealed an older bug which wouldn't have been exercized prior
to theme rooms:  a meditating vampire could and would shape change
without ceasing meditation.  Make it not shape change rather come out
of its trance.

src/mon.c

@@ -4476,7 +4476,7 @@ decide_to_shapeshift(struct monst *mon, int shiftflags)
         /* regular shapeshifter */
         if (!rn2(6))
             dochng = TRUE;
-    } else {
+    } else if (!(mon->mstrategy & STRAT_WAITFORU)) {
         /* The vampire has to be in good health (mhp) to maintain
          * its shifted form.
          *