Test failure of 'sgn_1' with musl libc

[hhoffstaette]

Gentoo CI noticed a test suite failure on musl

Out of curiosity I tried to reproduce it and indeed can do so (with 4.2.2 and also the latest 4.2.4), which is odd and probably a musl problem since the test suite of 4.2.2 passes on glibc :tada:

The test output summary says:

TEST SUMMARY
ERROR: number of missing images: 29
skipped images: ct.pnm cr.pnm sar2.pnm sar1.pnm mat.pnm mat.pnm mat.pnm sar1.pnm target.pnm target.pnm target.pnm target.pnm target.pnm target.pnm target.pnm target.pnm target.pnm target.pnm target.pnm target.pnm target.pnm target.pnm target.pnm target.pnm target.pnm target.pnm target.pnm target.pnm target.pnm
WARNING: skipped 38 tests
lossless_5 lossless_6 lossless_7 lossless_8 lossless_9 lossy_7 lossy_8 lossy_9 prg_2 prg_3 prg_4 prg_7 prg_8 prg_9 csty_0 csty_1 csty_2 csty_3 csty_4 csty_5 csty_6 csty_7 csty_8 csty_9 csty_10 csty_11 csty_12 csty_13 csty_14 ilyr_600 ilyr_605 ilyr_606 ilyr_607 ilyr_608 ilyr_609 roi_0 roi_1 roi_2
Number of tests: 91
Number of errors: 1
Failed tests:
    sgn_1.encode
STATUS: FAIL

and the specific test is:

TESTID=sgn_1 ENC=jasper DEC=jasper
FMT=pgx WIDTH=8 HEIGHT=8 PREC=12 NUMCMPTS=1 RAWSIZE=96
test information:
    image            signed_12.pgx   
    id               sgn_1           
    pae              0               
    bug              oj_dec          
    fmt              jpc             
    mode             int             
software=jasper input=/var/tmp/portage/media-libs/jasper-4.2.4/work/jasper-version-4.2.4/data/images/signed_12.pgx output=/tmp/jasper-@localhost/jpcod-171/test.jpc fmt=jpc mode=int verbose
Running JasPer encoder
/var/tmp/portage/media-libs/jasper-4.2.4/work/jasper-version-4.2.4_build/src/app/jasper  -T jpc -O cblkwidth=64 -O cblkheight=64 -O mode=int -O numrlvls=6 -O numgbits=2 -O prg=lrcp < /var/tmp/portage/media-libs/jasper-4.2.4/work/jasper-version-4.2.4/data/images/signed_12.pgx > /tmp/jasper-@localhost/jpcod-171/test.jpc
error: too few guard bits (2 < 20)
cannot encode main body
jpc_encode failed
error: cannot encode image
ERROR: ENCODER FAILURE
ERROR: ENCODER FAILURE

Looking at the test arguments, it does pass numgbits=2 and then complains about that, so it's not clear what is going here or why it succeeds on glibc.

Any idea what the problem might be? I can provide the full log if necessary.