Double-adding the image caused imlib's reference count to increment
twice, requiring us to free it twice to actually release the cached
image, but we only ever free it once.
Fixes#704, likely #721 (leak is not present with -DENABLE_RSVG=OFF),
possibly #650 based on connection to execp, maybe others since this was
introduced nearly 6 years ago in 1d02b858 in launcher-specific code.
