Fix segfault in compare_operators()

Add a NULL check to handle invalid input: an operator like '*' can be
identified as unary if it's used incorrectly, which leads to get_op()
returning NULL.
Style check

* Limit lines to 79 characters, not 80.
* Use "//" C99 inline comments instead of "/* */" block comments, except
  for function descriptions.
* Use an explicit void in function declarations with no arguments.
* tests.c: Rename "pSuite" to "suite", put "== NULL" after the value,
  not before.