patch 9.0.2022: getmousepos() returns wrong index for TAB char

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>
diff --git a/src/mouse.c b/src/mouse.c
index f3342f9..fe5c14c 100644
--- a/src/mouse.c
+++ b/src/mouse.c
@@ -3201,7 +3201,7 @@
 	|| defined(FEAT_EVAL) || defined(PROTO)
 /*
  * Convert a virtual (screen) column to a character column.
- * The first column is one.
+ * The first column is zero.
  */
     int
 vcol2col(win_T *wp, linenr_T lnum, int vcol)
@@ -3214,7 +3214,10 @@
     init_chartabsize_arg(&cts, wp, lnum, 0, line, line);
     while (cts.cts_vcol < vcol && *cts.cts_ptr != NUL)
     {
-	cts.cts_vcol += win_lbr_chartabsize(&cts, NULL);
+	int size = win_lbr_chartabsize(&cts, NULL);
+	if (cts.cts_vcol + size > vcol)
+	    break;
+	cts.cts_vcol += size;
 	MB_PTR_ADV(cts.cts_ptr);
     }
     clear_chartabsize_arg(&cts);