Fix overflow if history size is integer multiple of entries

[f-adler]

Thanks for the great library! I'm really happy with it. However, I came across an edge case which led to a crash in my system.

The issue can be reproduced with the following use case:

• Default history size: MICRORL_CFG_RING_HISTORY_LEN 64
• Commands send to history: Display.Backlight(0), Display.Backlight(1), Display.Backlight(2), Display.Backlight(3).
• Sending the up-key to select the last history entry subsequently crashed my program

Analysis:

• Contents of the commands don't matter, the error is seemingly triggered by the length of the commands leading to the trailing \0 being the last byte of the buffer.
• There is an out-of-bounds situation on this variable

  ++idx;                                      /* Move position from `\0` marker */
  

  (see diff)
• Crash happened here:

  size_t part0 = MICRORL_ARRAYSIZE(rbuf_ptr->ring_buf) - idx;
  memcpy(line_str, rbuf_ptr->ring_buf + idx, part0);
  

  with a negative part0 leading to memory overwrites.

The proposed code changes fix the error but additional consequences are not entirely clear. Feedback always welcome.

Cheers!
Felix