st/mesa: a couple fixes for st_BlitFramebuffer()
st/mesa: a couple fixes for st_BlitFramebuffer()

1. Loop over multiple destination color buffers.  If we set
glDrawBuffers(GL_FRONT_AND_BACK) we need to loop over multiple color
buffers, blitting to each.

2. Add checks for null src/dst surface pointers.  This fixes a crash
in the piglit fbo-missing-attachment-blit test.
See bug http://bugs.freedesktop.org/show_bug.cgi?id=59450

Reviewed-by: Reviewed-by: Marek Olšák <74bac9a67a04041c3586cda013cc5a11352e67be@gmail.com>

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