st/mesa: fix texture memory allocation bug

The following example caused an incorrect GL_OUT_OF_MEMORY error to be
raised in glTexSubImage2D:

   glTexImage2D(level=0, width=32, height=32, pixels=NULL);
   glTexImage2D(level=0, width=64, height=64, pixels=NULL);
   glTexSubImage2D(level=0, pixels!=NULL);

The second glTexImage2D() call needs to cause the first image to be
deallocated then reallocated at the new size.  This was not happening
because we were testing for pixels==NULL too early.

src/mesa/state_tracker/st_cb_texture.c

@@ -661,8 +661,10 @@ st_TexImage(GLcontext * ctx,
 					   format, type,
 					   pixels, unpack, "glTexImage");
    }
-   if (!pixels)
-      return;
+
+   /* Note: we can't check for pixels==NULL until after we've allocated
+    * memory for the texture.
+    */
 
    /* See if we can do texture compression with a blit/render.
     */
@@ -673,6 +675,9 @@ st_TexImage(GLcontext * ctx,
                                    stImage->pt->format,
                                    stImage->pt->target,
                                    PIPE_TEXTURE_USAGE_RENDER_TARGET, 0)) {
+      if (!pixels)
+         goto done;
+
       if (compress_with_blit(ctx, target, level, 0, 0, 0, width, height, depth,
                              format, type, pixels, unpack, texImage)) {
          goto done;
@@ -714,6 +719,9 @@ st_TexImage(GLcontext * ctx,
       return;
    }
 
+   if (!pixels)
+      goto done;
+
    DBG("Upload image %dx%dx%d row_len %x pitch %x\n",
        width, height, depth, width * texelBytes, dstRowStride);