Get rid of duplicate assignment of a->avail in gc_new_arena.

js/src/jsgc.c

@@ -201,7 +201,7 @@ gc_new_arena(JSArenaPool *pool, size_t nbytes)
     a = pool->current;
 
     /* Reset a->avail to start at the flags split, aka the first thing page. */
-    p = a->avail = FIRST_THING_PAGE(a);
+    p = FIRST_THING_PAGE(a);
     split = pagep = (uint8 *) p;
     a->avail = FIRST_THING(p, nbytes);
     JS_ASSERT(a->avail >= p + sizeof(JSGCPageInfo));