mesa: fix inverted pbo test error at _mesa_GetnCompressedTexImageARB

It seems like a typo.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/main/texgetimage.c

@@ -884,7 +884,7 @@ _mesa_GetnCompressedTexImageARB(GLenum target, GLint level, GLsizei bufSize,
       return;
    }
 
-   if (_mesa_is_bufferobj(ctx->Pack.BufferObj) && !img) {
+   if (!_mesa_is_bufferobj(ctx->Pack.BufferObj) && !img) {
       /* not an error, do nothing */
       return;
    }