Bug 1562187 - (part 4) Move an assertion to it makes more sense r=jandem

Move this assertion so it's nearer to where its assuption is used.

Differential Revision: https://phabricator.services.mozilla.com/D37362

--HG--
extra : moz-landing-system : lando

js/src/gc/Nursery.cpp

@@ -1229,12 +1229,12 @@ void js::Nursery::clear() {
     chunk(currentChunk_)
         .poisonAfterEvict(position() - chunk(currentChunk_).start());
   }
-  MOZ_ASSERT(maxChunkCount() > 0);
 
   /*
    * Reset the start chunk & position if we're not in this zeal mode, or we're
    * in it and close to the end of the nursery.
    */
+  MOZ_ASSERT(maxChunkCount() > 0);
   if (!runtime()->hasZealMode(ZealMode::GenerationalGC) ||
       (runtime()->hasZealMode(ZealMode::GenerationalGC) &&
        currentChunk_ + 1 == maxChunkCount())) {