Problem: When clicking in the middle of a TAB, getmousepos() returns
the column of the next char instead of the TAB.
Solution: Break out of the loop when the vcol to find is inside current
char. Fix invalid memory access when calling virtcol2col() on
an empty line.
closes: #13321
Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>