mesa: Don't look in unallocatd param slots for parameter values.

mesa: Don't look in unallocatd param slots for parameter values.

glsl-derivs would add 40.0, 0.0, and 1.0 in order.  When we went
looking for 0.0, we'd find it in the second slot of the param, and use
it, but param->Size would still be 1.  When we went to add 1.0 and
didn't find it, we'd put allocate it to that second slot and the 0.0
would actualy end up being 1.0.

Fixes glsl-derivs, glsl-deriv-varyings.