chrony plugin: fix 2 build warnings

  CC       src/chrony.lo
src/chrony.c:355:30: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
    if (!isalnum(c) || j + 1 >= p_buf_size)
                       ~~~~~ ^  ~~~~~~~~~~
src/chrony.c:359:9: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
  if (j < p_buf_size)
      ~ ^ ~~~~~~~~~~