Semicolon at the ends of if-statements and functions in C

that's dummy statememt. You sample is identical to

if (x == NULL) {
 // some code
 do_something_here();
}

/* empty (dummy statement) here */ ;

// more code
some_other_code_here();

They do nothing. They're a sign of someone who doesn't understand the language terribly well, I suspect.

If this is source code you notionally "own", I would remove the code and try to have a gentle chat with the person who wrote it.