st/mesa: Validate state before doing blits.

Specifically, this ensures things like the front buffer actually exist. This
fixes piglt fbo/fbo-sys-blit and fd.o bug 35483.

Signed-off-by: Henri Verbeet <hverbeet@gmail.com>

src/mesa/state_tracker/st_cb_blit.c

@@ -39,6 +39,7 @@
 #include "st_texture.h"
 #include "st_cb_blit.h"
 #include "st_cb_fbo.h"
+#include "st_atom.h"
 
 #include "util/u_blit.h"
 
@@ -75,6 +76,8 @@ st_BlitFramebuffer(struct gl_context *ctx,
    struct gl_framebuffer *readFB = ctx->ReadBuffer;
    struct gl_framebuffer *drawFB = ctx->DrawBuffer;
 
+   st_validate_state(st);
+
    if (!_mesa_clip_blit(ctx, &srcX0, &srcY0, &srcX1, &srcY1,
                         &dstX0, &dstY0, &dstX1, &dstY1)) {
       return; /* nothing to draw/blit */