st/mesa: Bail on incomplete attachments in discard_framebuffer
Incomplete attachments don't have an associated pipe_surface, so this would crash.
Fixes a WebGL conformance test that uses incomplete attachments: https://www.khronos.org/registry/webgl/sdk/tests/conformance2/renderbuffers/invalidate-framebuffer.html?webglVersion=2&quiet=0&quick=1
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111756