Bug 1108449 - Add nullptr check in ImageHost and CompositableHost. r=nical

gfx/layers/composite/CompositableHost.h

@@ -280,12 +280,12 @@ public:
   explicit AutoLockCompositableHost(CompositableHost* aHost)
     : mHost(aHost)
   {
-    mSucceeded = mHost->Lock();
+    mSucceeded = (mHost && mHost->Lock());
   }
 
   ~AutoLockCompositableHost()
   {
-    if (mSucceeded) {
+    if (mSucceeded && mHost) {
       mHost->Unlock();
     }
   }

gfx/layers/composite/ImageHost.cpp

@@ -250,6 +250,9 @@ bool
 ImageHost::Lock()
 {
   MOZ_ASSERT(!mLocked);
+  if (!mFrontBuffer) {
+    return false;
+  }
   if (!mFrontBuffer->Lock()) {
     return false;
   }
@@ -261,7 +264,9 @@ void
 ImageHost::Unlock()
 {
   MOZ_ASSERT(mLocked);
+  if (mFrontBuffer) {
     mFrontBuffer->Unlock();
+  }
   mLocked = false;
 }