Bug 1384561: Null-check pointer before deleting it (rather than after), in SVG ClearPreserveAspectRatioProperty() function. r=jwatt

In practice, this shouldn't make a difference -- but C++ allows compilers to
make pointer-usage-after-delete trigger a runtime fault.

MozReview-Commit-ID: J1tZ3TxXR80

--HG--
extra : rebase_source : 046407c3c034b85a5b5d41e4c1a87e4a79c6833e

dom/svg/SVGSVGElement.cpp

@@ -750,8 +750,9 @@ bool
 SVGSVGElement::ClearPreserveAspectRatioProperty()
 {
   void* valPtr = UnsetProperty(nsGkAtoms::overridePreserveAspectRatio);
+  bool didHaveProperty = !!valPtr;
   delete static_cast<SVGPreserveAspectRatio*>(valPtr);
-  return valPtr;
+  return didHaveProperty;
 }