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/normal.c b/src/normal.c
index c543635..f08f52f 100644
--- a/src/normal.c
+++ b/src/normal.c
@@ -1571,7 +1571,12 @@
 
 	    oap->start = VIsual;
 	    if (VIsual_mode == 'V')
+	    {
 		oap->start.col = 0;
+# ifdef FEAT_VIRTUALEDIT
+		oap->start.coladd = 0;
+# endif
+	    }
 	}
 
 	/*
@@ -7580,6 +7585,7 @@
     if (!virtual_active())
 	curwin->w_cursor.coladd = 0;
 #endif
+    check_cursor_col();
 #ifdef FEAT_FOLDING
     if (cap->oap->op_type == OP_NOP
 	    && pos != NULL