svga: fix a crash happening before setting fragment shaders.

svga: fix a crash happening before setting fragment shaders.

In certain situations API's will call pipe->clear which doesn't
require fragment shader, but then we'd try to verify the pipeline
and assume fragment shader was always set. This was leading to
crash when API would just call simple clear's before anything else.

NOTE: This is a candidate for the 8.0 branch.

Reviewed-by: José Fonseca <jfonseca@vmware.com>