Avoid crash with gzipped file

Avoid crash with gzipped file

In computing the length of the extension, the code does not account
for the prefix "unzipped_", which is added when the zipped filename
does not have one of the extensions "gz", "z", "Z", or "svgz", and
thus the used index is out of bounds. See also this thread:
https://www.mail-archive.com/lyx-devel@lists.lyx.org/msg207360.html