Fix #12638 FP overlappingWriteFunction with last array element

[chrchr-github]

No description provided.

[chrchr-github]

Seems like we have several incorrect test cases:

void foo() {
    wchar_t a[10];
    wmemcpy(&a[5], &a[4], 2u);
}

should not result in an error.

Edit: Actually it should, since wmemcpy() takes a character count instead of a byte count as the 3rd arg. This isn't evident from std.cfg though.