swrast: When reading from a 0-bits r,g,b channel, return 0 not 1.

swrast: When reading from a 0-bits r,g,b channel, return 0 not 1.

It looks like we were reading a fractional value, multiplying by an
enormous negative value, then stuffing that value into a bitfield
assuming it was already clamped.  This becomes relevant for GL_ALPHA
or R/RG FBOs.