fix#1379
decodecorpus was generating one extraneous byte when `nbSeq==0`.
This is disallowed by the specification.
The reference decoder was just skipping the extraneous byte.
It is now stricter, and flag such situation as an error.
gcc-8 on Linux doesn't like usage of strncat :
`warning: ‘strncat’ output truncated before terminating nul copying as many bytes from a string as its length`.
Not sure what was wrong, it might be a false positive,
but the logic is simple enough to replaced by a simple `memcpy()`,
thus avoiding the shenanigans of null-terminated strings.
following #1356,
only enable backtrace compilation on linux+glibc.
Also, disable backtrace by default from "release" compilation,
so that less platforms get impacted by the new requirements.
Can be manually enabled/disabled using BACKTRACE=1/0.
and slightly refactored affected function.
Honestly, the formula calculating variance should get a second reviewing round,
it's not clear if it's correct.
some non-trivial changes to platform.h and util.h,
initially related to compilation for Haiku,
but I used this opportunity to make them cleaner
and add some documentation.
Noticed several tests that could be improved
(too harsh conditions, useless exception, etc.)
but I did not dare modifying too many tests just before release.
note : for some reason,
scan-build version on my laptop found problems within fastcover.c
that scan-build on travisCI does not flag.
They are, as usual, false positive :
the analyzer does not understand that a table (`offset`) is correctly filled before usage.
for Linux and Mac OS-X.
Note : the backtraces fires up through a trap
before the sanitizer get a chance to report.
There are situations where the sanitizer report is actually preferable.
It might be good to consider a kind of build macro
which can disable backtrace
when sanitizer is enabled.
from EXIT_IF() to RETURN_IF()
EXIT could be misunderstood as exit(), which terminates program execution.
But the macro only leaves the function, not the program.
