bug#24688: 25.1.50; testcover-start error when instrumenting pcase pattern

[Gemini Lasswell]

testcover-start fails with the message "Wrong type argument: listp, \,"
when asked to instrument lisp/emacs-lisp/cconv.el from the Emacs source
tree. I've simplified the problem down to this fragment of code:

(pcase form
    (`(condition-case ,var ,protected-form . ,handlers)
     nil))

To reproduce, create a file called bug.el containing the code above and:
1. emacs -Q
2. M-x toggle-debug-on-error RET
3. C-x C-f bug.el RET
4. M-: (require 'testcover) RET
5. M-x testcover-start RET bug.el RET

Here is the resulting backtrace:

backtrace.txt
Description: Text document

In GNU Emacs 25.1.50.18 (x86_64-apple-darwin15.6.0, NS appkit-1404.47 Version 
10.11.6 (Build 15G1004))
 of 2016-10-12 built on rainbow.local
Windowing system distributor 'Apple', version 10.3.1404
Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
testcover
mapcar: Wrong type argument: listp, \,
Debug on Error enabled globally
You can run the command ‘toggle-debug-on-error’ with M-x t-er RET
Debug on Error enabled globally
Entering debugger...

Configured using:
 'configure --with-ns'

Configured features:
JPEG RSVG IMAGEMAGICK NOTIFY ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS
NS

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Debugger

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message subr-x puny seq byte-opt gv
bytecomp byte-compile cl-extra cconv dired dired-loaddefs format-spec
rfc822 mml mml-sec password-cache epa derived epg epg-config gnus-util
rmail rmail-loaddefs mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mail-utils help-mode debug cus-start cus-load
testcover edebug easymenu cl-loaddefs pcase cl-lib time-date mule-util
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel term/ns-win ns-win ucs-normalize term/common-win tool-bar dnd
fontset image regexp-opt fringe tabulated-list newcomment elisp-mode
lisp-mode prog-mode register page menu-bar rfn-eshadow timer select
scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame cl-generic cham georgian utf-8-lang misc-lang
vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms cp51932
hebrew greek romanian slovak czech european ethiopic indian cyrillic
chinese charscript case-table epa-hook jka-cmpr-hook help simple abbrev
obarray minibuffer cl-preloaded nadvice loaddefs button faces cus-face
macroexp files text-properties overlay sha1 md5 base64 format env
code-pages mule custom widget hashtable-print-readable backquote kqueue
cocoa ns multi-tty make-network-process emacs)

Memory information:
((conses 16 216154 8450)
 (symbols 48 21704 0)
 (miscs 40 64 189)
 (strings 32 20892 6047)
 (string-bytes 1 644659)
 (vectors 16 35191)
 (vector-slots 8 672545 4247)
 (floats 8 184 26)
 (intervals 56 239 0)
 (buffers 976 13))