Bug 1030667 - Don't pass 0 to realloc in ArrayBufferBuilder::setCapacity. r=sfink

dom/base/nsXMLHttpRequest.cpp

@@ -3973,7 +3973,10 @@ ArrayBufferBuilder::setCapacity(uint32_t aNewCap)
 {
   MOZ_ASSERT(!mMapPtr);
 
-  uint8_t *newdata = (uint8_t *) js_realloc(mDataPtr, aNewCap);
+  // To ensure that realloc won't free mDataPtr, use a size of 1
+  // instead of 0.
+  uint8_t* newdata = (uint8_t *) js_realloc(mDataPtr, aNewCap ? aNewCap : 1);
+
   if (!newdata) {
     return false;
   }