symbols: Fix NULL pointer dereference in MergeKeyGroupsHEADmaster

It may trigger with the following keymap symbols:

    key <> { [a, A] }; key <> { [NoAction()] };

or:

    key <> { [NoAction()] }; augment key <> { [a, A] };

Part-of: <https://gitlab.freedesktop.org/xorg/app/xkbcomp/-/merge_requests/33>

1 files changed, 11 insertions, 0 deletions

diff --git a/symbols.c b/symbols.c
index be306b6..d6c1907 100644
--- a/symbols.c
+++ b/symbols.c
@@ -414,6 +414,17 @@ MergeKeyGroups(SymbolsInfo * info,
             {
                 resultActs[i] = *fromAct;
             }
+            else if (toAct == NULL && fromAct == NULL)
+            {
+                /*
+                 * May happen with e.g.:
+                 *
+                 *     key <> { [a, A] }; key <> { [NoAction()] };
+                 * or:
+                 *     key <> { [NoAction()] }; augment key <> { [a, A] };
+                 */
+                resultActs[i].type = XkbSA_NoAction;
+            }
             else
             {
                 XkbAction *use, *ignore;