Fix potential overflow in fpc_chararray_to_shortstr for 8 bit CPU

Summary

Fix potential overflow in fpc_chararray_to_shortstr for an 8 bit CPU.

System

• Operating system: embedded
• Processor architecture: AVR
• Device: Microcontroller

What is the current bug behavior?

Converting an array of char to a shortstring leads to incorrect data being copied if the shortstring length is 255.

What is the behavior after applying this patch?

Correct conversion of array of char to shortstring

Test case

program test_shortstrings;

const
  charArray: array[0..8] of char = 'charArray';
  shortStr: shortstring = 'shortStr';

var
  stemp: shortstring;

begin
  stemp := charArray;
  writeln(stemp);
  writeln(shortStr);
end.

Output:

charArrashortStr
shortStr

Output after fix:

charArray
shortStr