Possible remaining gzip compression errors

[jodavies]

The main issue of #95 was fixed long ago in 26793e4 and the separate issue (when form is compiled without zlib) discovered during debugging investigations was fixed in 68c8a8a .

But there have been two reports in #95 which have a similar looking crash, but a different error code from zlib: -3 rather than -2:
FillInputGZIP: Error in gzip handling of input. zerror = -3

I have attempted to reproduce this crash using code from both reports, but have not been able to.

If this error ever comes up again (or existing reports can be reproduced one day) let's discuss it here.