fmt/print: fix cppcheck overflow warning

[sreimers]

src/fmt/print.c:77:15: error: Pointer arithmetic overflow; 'buf' buffer size is 64 [ctuPointerArith]
 char c, *p = buf + NUM_SIZE - 1;
              ^
src/fmt/print.c:272:20: note: Calling function local_itoa, 1st argument is uninitialized
   len = local_itoa(num, (sn < 0) ? -sn : sn, base,
                   ^
src/fmt/print.c:77:15: note: Using argument buf
 char c, *p = buf + NUM_SIZE - 1;