vim-patch:9.0.2022: getmousepos() returns wrong index for TAB char (#25636)

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: vim/vim#13321

b583eda703

src/nvim/eval.lua

@@ -12107,6 +12107,8 @@ M.funcs = {
       character in window {winid} at buffer line {lnum} and virtual
       column {col}.
 
+      If buffer line {lnum} is an empty line, 0 is returned.
+
       If {col} is greater than the last virtual column in line
       {lnum}, then the byte index of the character at the last
       virtual column is returned.