[bug-ccd2cue] assert on negative index that indicates pregap

[Sean Bailey]

Greetings,

An old (2008) .ccd file that I have uses "INDEX 0=-150" to specify a 2
second pregap for the first track. The result is "ccd2cue:
convert.c:83: frames2msf: Assertion `frames >= 0' failed. Aborted
(core dumped)".

IMHO, rather than the assertion, an appropriate PREGAP command should
be output to the .cue file. Section B.6.3 of the documentation would
also need correcting. (And might I say, that documentation really puts
the Fine in RTFM.)

I have no idea if the age of the .ccd file has any bearing on the
issue, but thought it best to mention it.

Using the Arch ccd2cue 0.5-1 package on Linux ArchDesktop 4.9.41-1-lts
#1 SMP Mon Aug 7 17:32:35 CEST 2017 x86_64 GNU/Linux. Invocation
"ccd2cue -o example\ file.cue example\ file.ccd".

Cheers,
Sean