[LyX/master] Fix pasting of math grid with more rows in source than target (#11897)

[Juergen Spitzmueller]

commit 16d0785e6707f98289d27b7364d0ecb82de0a697
Author: Juergen Spitzmueller <spitz at lyx.org>
Date:   Mon Jun 29 09:48:05 2020 +0200

    Fix pasting of math grid with more rows in source than target (#11897)
---
 src/mathed/InsetMathGrid.cpp |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/src/mathed/InsetMathGrid.cpp b/src/mathed/InsetMathGrid.cpp
index d6f275a..52283b5 100644
--- a/src/mathed/InsetMathGrid.cpp
+++ b/src/mathed/InsetMathGrid.cpp
@@ -1651,8 +1651,9 @@ void InsetMathGrid::doDispatch(Cursor & cur, FuncRequest & cmd)
 			// append the left over vertical cells to the last _cell_
 			idx_type i = nargs() - 1;
 			for (row_type r = numrows; r < grid.nrows(); ++r) {
+				addRow(r - 1);
 				for (col_type c = 0; c < grid.ncols(); ++c)
-					cell(i).append(grid.cell(grid.index(r, c)));
+					cell(index(r, c)).append(grid.cell(grid.index(r, c)));
 				if (hline_enabled)
 					rowinfo_[r].lines += grid.rowinfo_[r].lines;
 				else {