swrast: fix a few release build warnings

src/mesa/swrast/s_context.c

@@ -251,6 +251,7 @@ _swrast_update_fog_state( struct gl_context *ctx )
    const struct gl_fragment_program *fp = ctx->FragmentProgram._Current;
 
    assert(fp == NULL || fp->Base.Target == GL_FRAGMENT_PROGRAM_ARB);
+   (void) fp; /* silence unused var warning */
 
    /* determine if fog is needed, and if so, which fog mode */
    swrast->_FogEnabled = (!_swrast_use_fragment_program(ctx) &&

src/mesa/swrast/s_span.c

@@ -1468,6 +1468,7 @@ _swrast_read_rgba_span( struct gl_context *ctx, struct gl_renderbuffer *rb,
 	     rb->_BaseFormat == GL_ALPHA);
 
       assert(srb->Map);
+      (void) srb; /* silence unused var warning */
 
       src = _swrast_pixel_address(rb, x + skip, y);