mesa: Return -FLT_MAX instead of 0 for LG2(0).

lim x->0 log(x) = -inf so -FLT_MAX is a better approximation than 0
for LG2(0).

src/mesa/shader/prog_execute.c

@@ -939,7 +939,7 @@ _mesa_execute_program(GLcontext * ctx,
 	    /* The fast LOG2 macro doesn't meet the precision requirements.
 	     */
             if (a[0] == 0.0F) {
-               val = 0.0F;
+               val = -FLT_MAX;
             }
             else {
                val = log(a[0]) * 1.442695F;