Fix DiscardableMemoryPool::free race condition

src/lazy/SkDiscardableMemoryPool.cpp

Index: src/lazy/SkDiscardableMemoryPool.cpp
diff --git a/src/lazy/SkDiscardableMemoryPool.cpp b/src/lazy/SkDiscardableMemoryPool.cpp
index 1f4836604d2233914af709a7ce06d5f1db82d88c..5b9d87ff2fd1d4a9222d156b365a187efd5ec968 100644
--- a/src/lazy/SkDiscardableMemoryPool.cpp
+++ b/src/lazy/SkDiscardableMemoryPool.cpp
@@ -188,9 +188,9 @@ SkDiscardableMemory* DiscardableMemoryPool::create(size_t bytes) {
 }
 
 void DiscardableMemoryPool::free(PoolDiscardableMemory* dm) {
+    SkAutoMutexAcquire autoMutexAcquire(fMutex);
     // This is called by dm's destructor.
     if (dm->fPointer != NULL) {
-        SkAutoMutexAcquire autoMutexAcquire(fMutex);
         sk_free(dm->fPointer);
         dm->fPointer = NULL;
         SkASSERT(fUsed >= dm->fBytes);