If char is unsigned (as on ARM) subtracting a larger number would
result in a wrap around, not a negative value.

  src/nnn.c: In function 'readpipe':
  src/nnn.c:4325:11: warning: comparison is always false due to limited
  range of data type [-Wtype-limits]

     if (ctx < 0 || ctx > CTX_MAX)
             ^