vim-patch:8.1.2321: cannot select all text with the mouse

Problem: Cannot select all text with the mouse. (John Marriott)
Solution: Move limiting the mouse column to f_getmousepos().
          (closes https://github.com/vim/vim/issues/5242)
0a5aa7b28a

src/nvim/eval/funcs.c

@@ -3780,7 +3780,18 @@ void f_getmousepos(typval_T *argvars, typval_T *rettv, FunPtr fptr)
       winrow = row + 1 + wp->w_border_adj[0];  // Adjust by 1 for top border
       wincol = col + 1 + wp->w_border_adj[3];  // Adjust by 1 for left border
       if (row >= 0 && row < wp->w_height && col >= 0 && col < wp->w_width) {
+        char_u *p;
+        int count;
+
         mouse_comp_pos(wp, &row, &col, &line);
+
+        // limit to text length plus one
+        p = ml_get_buf(wp->w_buffer, line, false);
+        count = (int)STRLEN(p);
+        if (col > count) {
+          col = count;
+        }
+
         column = col + 1;
       }
     }