Bug 1470355 - BufferWriter should stop reading data when the wanted amount has been reached, r=michal

netwerk/base/nsNetUtil.cpp

@@ -1674,6 +1674,11 @@ private:
                 if (mCount != -1) {
                     MOZ_ASSERT(mCount >= writtenData);
                     mCount -= writtenData;
+
+                    // Is this the end of the reading?
+                    if (mCount == 0) {
+                        break;
+                    }
                 }
 
                 continue;