Fix for -Wstrict-overflow=3

[michaelortmann]

Found by: https://github.com/michaelortmann
Patch by: https://github.com/michaelortmann
Fixes:

One-line summary:
Fix for -Wstrict-overflow=3

Additional description (if needed):

gcc -fPIC -std=gnu99 -Wstrict-overflow=3 -pipe -Wall -I. -I../../.. -I../../.. -I../../../src/mod  -DHAVE_CONFIG_H -I/usr/include  -DMAKING_MODS -c .././filesys.mod/filesys.c && mv -f filesys.o ../
.././filesys.mod/filesys.c: In function ‘mktempfile’:
.././filesys.mod/filesys.c:714:3: warning: assuming signed overflow does not occur when changing X +- C1 cmp C2 to X cmp C2 -+ C1 [-Wstrict-overflow]
  714 |   if ((l + MKTEMPFILE_TOT) > NAME_MAX) {
      |   ^~

Test cases demonstrating functionality (if applicable):