Add null terminator to string literals in x86 module and fix a bug that interfered with use of function name labels in call instructions

#{
fib <- :n {
  if: n < 2 {
    1
  } else: {
    (fib: n-1) + (fib: n-2)
  }
}

main <- {
  print: (string: (fib: 30)) . "\n"
}

}