apply proposed fix for the buffer overrun

This MR resolves 1067787

Changes:

• apply proposed fix from https://github.com/linbox-team/linbox/pull/307

Testing:

• build with _FORTIFY_SOURCE=3 - https://launchpad.net/~vpa1977/+archive/ubuntu/october-21/+sourcepub/15875163/+listing-archive-extra
• autopkgtest:

autopkgtest [08:49:23]: test command1: -----------------------]
autopkgtest [08:49:24]: test command1:  - - - - - - - - - - results - - - - - - - - - -
command1             PASS
autopkgtest [08:49:24]: @@@@@@@@@@@@@@@@@@@@ summary
command1             PASS