gmarkup: Fix crash in error handling path for closing elements

If something which looks like a closing tag is left unfinished, but
isn’t paired to an opening tag in the document, the error handling code
would do a null pointer dereference. Avoid that, at the cost of
introducing a new translatable error message.

Includes a test case, courtesy of pdknsk.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://gitlab.gnome.org/GNOME/glib/issues/1461
(cherry picked from commit fccef3cc822af74699cca84cd202719ae61ca3b9)

glib/gmarkup.c

@@ -1844,9 +1844,14 @@ g_markup_parse_context_end_parse (GMarkupParseContext  *context,
     case STATE_AFTER_CLOSE_TAG_SLASH:
     case STATE_INSIDE_CLOSE_TAG_NAME:
     case STATE_AFTER_CLOSE_TAG_NAME:
-      set_error (context, error, G_MARKUP_ERROR_PARSE,
-                 _("Document ended unexpectedly inside the close tag for "
-                   "element '%s'"), current_element (context));
+      if (context->tag_stack != NULL)
+        set_error (context, error, G_MARKUP_ERROR_PARSE,
+                   _("Document ended unexpectedly inside the close tag for "
+                     "element '%s'"), current_element (context));
+      else
+        set_error (context, error, G_MARKUP_ERROR_PARSE,
+                   _("Document ended unexpectedly inside the close tag for an "
+                     "unopened element"));
       break;
 
     case STATE_INSIDE_PASSTHROUGH:

glib/tests/Makefile.am

@@ -156,6 +156,7 @@ markup_tests = \
 	fail-36 fail-37 fail-38 fail-39 fail-40 \
 	fail-41 fail-42 fail-43 fail-44 fail-45 \
 	fail-46 fail-47 fail-48 fail-49 fail-50 \
+	fail-51 \
 	valid-1 valid-2 valid-3 valid-4 valid-5 \
 	valid-6 valid-7 valid-8 valid-9 valid-10 \
 	valid-11 valid-12 valid-13 valid-14 valid-15 \

glib/tests/markups/fail-51.expected

@@ -0,0 +1 @@
+ERROR Error on line 1 char 5: Document ended unexpectedly inside the close tag for an unopened element

glib/tests/markups/fail-51.gmarkup

@@ -0,0 +1 @@
+</0<