HANDLE_FATAL_SIGNAL IN STRNLEN

Fixed the following bounds checking problems :
1. in check_if_legal_filename() make sure the null terminated
string is long enough before accessing the bytes in it.
Prevents pottential read-past-buffer-end
2. in my_wc_mb_filename() of the filename charset check
for the end of the destination buffer before sending single
byte characters into it.
Prevents write-past-end-of-buffer (and garbaling stack in
the cases reported here) errors.

Added test cases.