Don't traverse linked list when saving a marker

If the calling application invokes jpeg_save_markers() to save a
particular type of marker, then the save_marker() function will be
invoked for every marker of that type that is encountered.  Previously,
only the head of the marker linked list was stored (in
jpeg_decompress_struct), so save_marker() had to traverse the entire
linked list before it could add a new marker to the tail of the list.
That caused CPU usage to grow exponentially with the number of markers.
Referring to #764, it is possible to create a JPEG image that contains
an excessive number of markers.  The specific reproducer that uncovered
this issue is a specially-crafted 1-megabyte malformed JPEG image with
tens of thousands of APP1 markers, which required approximately 30
seconds of CPU time (on a modern Intel processor) to process.  However,
it should also be possible to create a legitimate JPEG image that
reproduces the issue (such as an image with tens of thousands of
duplicate EXIF tags.)

This commit introduces a new pointer (in jpeg_decomp_master, in order to
preserve backward ABI compatibility) that is used to store the tail of
the marker linked list whenever a marker is added to it.  Thus, it is no
longer necessary to traverse the list when adding a marker, and CPU
usage will grow linearly rather than exponentially with the number of
markers.

Fixes #764

jcomapi.c

@@ -3,8 +3,8 @@
  *
  * This file was part of the Independent JPEG Group's software:
  * Copyright (C) 1994-1997, Thomas G. Lane.
- * It was modified by The libjpeg-turbo Project to include only code relevant
- * to libjpeg-turbo.
+ * libjpeg-turbo Modifications:
+ * Copyright (C) 2024, D. R. Commander.
  * For conditions of distribution and use, see the accompanying README.ijg
  * file.
  *
@@ -51,6 +51,7 @@ jpeg_abort(j_common_ptr cinfo)
      * A bit kludgy to do it here, but this is the most central place.
      */
     ((j_decompress_ptr)cinfo)->marker_list = NULL;
+    ((j_decompress_ptr)cinfo)->master->marker_list_end = NULL;
   } else {
     cinfo->global_state = CSTATE_START;
   }