mesa: raise GL_INVALID_OPERATION in glGenerateMipmap for missing base image

This seems to be expected by the WebGL texture-mips test.  The error makes
sense, but I haven't found (yet) any OpenGL documentation specifying this
error condition.

See http://bugs.freedesktop.org/show_bug.cgi?id=44912

Note: This is a candidate for the 8.0 branch.

Reviewed-by: José Fonseca <jfonseca@vmware.com>

src/mesa/main/fbobject.c

@@ -2564,6 +2564,8 @@ _mesa_GenerateMipmapEXT(GLenum target)
    srcImage = _mesa_select_tex_image(ctx, texObj, target, texObj->BaseLevel);
    if (!srcImage) {
       _mesa_unlock_texture(ctx, texObj);
+      _mesa_error(ctx, GL_INVALID_OPERATION,
+                  "glGenerateMipmap(zero size base image)");
       return;
    }