app: do not try to copy a buffer result from a canceled GEGL operation.
This fixes: > GEGL-WARNING: (../../src/gegl/gegl/buffer/gegl-tile.c:127):gegl_tile_dup: runtime check failed: (! src->damage) Which happened when a GEGL operation was running and you canceled it in the middle, say with the ESC key (easy to reproduce with long operations such as "Color to Gray"). In such case, obviously don't try to copy the unfinished operation result into the dest buffer.
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