md_analyze_tilde: Pop from chain tail like other emphasis.

The function incorrectly used header from the head, leading to wrong
result (incompatible with e.. GFM) but even worse to bad internal state
md_rollback() is then potentially unable to solve.

Fixes #222.

src/md4c.c

@@ -3836,8 +3836,8 @@ md_analyze_tilde(MD_CTX* ctx, int mark_index)
      * only tildes sequences of length 1 and 2, and the length of the opener
      * and closer has to match. */
 
-    if((mark->flags & MD_MARK_POTENTIAL_CLOSER)  &&  chain->head >= 0) {
-        int opener_index = chain->head;
+    if((mark->flags & MD_MARK_POTENTIAL_CLOSER)  &&  chain->tail >= 0) {
+        int opener_index = chain->tail;
 
         md_rollback(ctx, opener_index, mark_index, MD_ROLLBACK_CROSSING);
         md_resolve_range(ctx, chain, opener_index, mark_index);

test/regressions.txt

@@ -672,3 +672,14 @@ _*xx-_-
 <p><em>*__-</em>-</p>
 <p><em>*xx-</em>-</p>
 ````````````````````````````````
+
+
+## [Issue 222](https://github.com/mity/md4c/issues/222)
+
+```````````````````````````````` example
+~foo ~bar baz~
+.
+<p>~foo <del>bar baz</del></p>
+.
+--fstrikethrough
+````````````````````````````````