Bug 1174046 - Fix PLDHashTable::Iterator in chaos mode again. r=froydnj.

If you use PLDHashTable::Iterator in chaos mode with a table with zero
capacity, a |% 0| operation takes place in randomUint32LessThan. This change
avoids that.

--HG--
extra : rebase_source : 85f2affb57c2402f40f3d117434b8300e7f204b7

mfbt/ChaosMode.h

@@ -81,6 +81,7 @@ public:
    */
   static uint32_t randomUint32LessThan(uint32_t aBound)
   {
+    MOZ_ASSERT(aBound != 0);
     return uint32_t(rand()) % aBound;
   }
 };

xpcom/glue/pldhash.cpp

@@ -910,7 +910,7 @@ PLDHashTable::Iterator::Iterator(const PLDHashTable* aTable)
   // mEntryAddr, respectively.
   uint32_t capacity = mTable->Capacity();
 
-  if (ChaosMode::isActive(ChaosMode::HashTableIteration)) {
+  if (ChaosMode::isActive(ChaosMode::HashTableIteration) && capacity > 0) {
     // Start iterating at a random point in the hashtable. It would be
     // even more chaotic to iterate in fully random order, but that's a lot
     // more work.