swrast: return (0,0,0,1) when sampling incomplete textures, not (0,0,0,0)

Fixes piglit shaders/fp-incomplete-tex test.

src/mesa/swrast/s_fragprog.c

@@ -84,7 +84,8 @@ fetch_texel_lod( GLcontext *ctx, const GLfloat texcoord[4], GLfloat lambda,
       swizzle_texel(rgba, color, texObj->_Swizzle);
    }
    else {
-      color[0] = color[1] = color[2] = color[3] = 0.0F;
+      color[0] = color[1] = color[2] = 0.0F;
+      color[3] = 1.0F;
    }
 }
 
@@ -126,7 +127,8 @@ fetch_texel_deriv( GLcontext *ctx, const GLfloat texcoord[4],
       swizzle_texel(rgba, color, texObj->_Swizzle);
    }
    else {
-      color[0] = color[1] = color[2] = color[3] = 0.0F;
+      color[0] = color[1] = color[2] = 0.0F;
+      color[3] = 1.0F;
    }
 }