static analyzer bit for trap.c

src/trap.c(5794): warning: Dereferencing NULL pointer 'which'.

src/trap.c

@@ -5791,6 +5791,7 @@ openholdingtrap(
     if (!which)
         which = t->tseen ? the_your[t->madeby_u]
                          : strchr(vowels, *trapdescr) ? "an" : "a";
+    assert(which != 0);
     if (*which)
         which = strcat(strcpy(whichbuf, which), " ");