[runtime] Fix a leak when a cast from an existential fails.

rdar://25011739.

Author: Greg Parker

stdlib/public/runtime/Casting.cpp

@@ -1592,12 +1592,18 @@ static bool _dynamicCastFromExistential(OpaqueValue *dest,
 
   bool result = swift_dynamicCast(dest, srcValue, srcCapturedType,
                                   targetType, subFlags);
-  // Deallocate the existential husk if we took from it.
-  if (canTake && result && isOutOfLine)
-    _maybeDeallocateOpaqueExistential(src, result, flags);
-  // If we couldn't take, we still may need to destroy the whole value.
-  else if (!canTake && shouldDeallocateSource(result, flags))
-    srcType->vw_destroy(src);
+
+  if (!canTake) {
+    // swift_dynamicCast performed no memory management.
+    // Destroy the value if requested.
+    if (shouldDeallocateSource(result, flags))
+      srcType->vw_destroy(src);
+  } else {
+    // swift_dynamicCast took or destroyed the value as per the original request
+    // We may still have an opaque existential container to deallocate.
+    if (isOutOfLine)
+      _maybeDeallocateOpaqueExistential(src, result, flags);
+  }
 
   return result;
 }