negf: Fix -Wformat-truncation warning
math.c: In function ‘negf’:
math.c:474:36: warning: ‘snprintf’ output may be truncated before the last format character [-Wformat-truncation=]
474 | snprintf(tmp, sizeof(tmp), "-%s", dispstr);
| ^
math.c:474:5: note: ‘snprintf’ output between 2 and 33 bytes into a
destination of size 32
474 | snprintf(tmp, sizeof(tmp), "-%s", dispstr);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~