fix isl_printer_start_line for string printer
The string printer failed to terminate the string buffer
after printing the indentation.
This does not usually cause any problems because
the call to isl_printer_start_line is typically followed
by a call to some other printing.
However, if isl_printer_get_str gets called immediately
after a call to isl_printer_start_line, it could potentially
read uninitialized memory.
Terminate the string after printing the indentation.
Signed-off-by: Sven Verdoolaege <sven@cerebras.net>