Fix fpcrtl_strappendA, which cut last char from UserPathPrefix leading to PHYSFS_setWriteDir fail with 'File not found' error, which led to lack of writing dir and logFile handle being nil, which led to crash on attempt to write to (absent) log file

[patterns]
**.h = native
**.c = native
**.cpp = native
**.lua = native
**.txt = native
**.pas = native
**.inc = native
**.hs = native