patch 8.0.0962: crash with virtualedit and joining lines

Problem:    Crash with virtualedit and joining lines. (Joshua T Corbin, Neovim
            #6726)
Solution:   When using a mark check that coladd is valid.
diff --git a/src/Makefile b/src/Makefile
index de4b240..1f252f0 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2274,6 +2274,7 @@
 	test_utf8_comparisons \
 	test_viminfo \
 	test_vimscript \
+	test_virtualedit \
 	test_visual \
 	test_window_cmd \
 	test_window_id \