Bug 1050035 (part 4) - Make PL_DHashTableAdd() infallible by default, and add a fallible alternative. r=froydnj.

I kept all the existing PL_DHashTableAdd() calls fallible, in order to be
conservative, except for the ones in nsAtomTable.cpp which already were
followed immediately by an abort on failure.

embedding/components/commandhandler/nsCommandParams.cpp

@@ -230,7 +230,8 @@ nsCommandParams::GetOrMakeEntry(const char* aName, uint8_t entryType)
     return foundEntry;
   }
 
-  foundEntry = (HashEntry *)PL_DHashTableAdd(&mValuesHash, (void *)aName);
+  foundEntry = static_cast<HashEntry*>
+    (PL_DHashTableAdd(&mValuesHash, (void *)aName, fallible));
   if (!foundEntry) {
     return nullptr;
   }