Fix error trap in SurfaceTexture Client

There was an issue in Surface::lock where failure to lock a surface
resulted in two bad things happening:
- success was returned to the caller (it was apparently locked).
- an uninitialised pointer was returned as the buffer.

Change-Id: I8b0df81400e0fa0542a8bb993d76923ac96b686e

libs/gui/SurfaceTextureClient.cpp

@@ -757,6 +757,9 @@ status_t SurfaceTextureClient::lock(
             ALOGW_IF(res, "failed locking buffer (handle = %p)",
                     backBuffer->handle);
 
+            if (res != 0) {
+                err = INVALID_OPERATION;
+            } else {
                 mLockedBuffer = backBuffer;
                 outBuffer->width  = backBuffer->width;
                 outBuffer->height = backBuffer->height;
@@ -765,6 +768,7 @@ status_t SurfaceTextureClient::lock(
                 outBuffer->bits   = vaddr;
             }
         }
+    }
     return err;
 }