Fix control-flow handling of modifiers without body.

[chriseth]

Fixes #10620

I'm still a bit puzzled why the bug does not manifest in

abstract contract A {
    function f() public view mod {
        //uint x = 2;
        require(block.timestamp > 10);
    }
    modifier mod() virtual;
}

(the require statement is essential for the bug to trigger)

So maybe this is not the right fix.

[ekpyron]

Could get a bugfix changelog entry, but the fix is fine in any case.