st/mesa: check image size before copy_image_data_to_texture()

st/mesa: check image size before copy_image_data_to_texture()

We should only copy images into the dest texture if the size is correct.
This fixes a failed assertion when finalizing a texture with mis-defined
mipmap levels such as:
level 0: 32x32
level 1: 8x8

Also, fix incorrect mipmap level used in assertion at the top of
copy_image_data_to_texture().

NOTE: This is a candidate for the 7.10 branch.