Fix ‘!NILP (Vpurify_flag)’ assertion failure during temacs bootstrap

The recent changes to src/casefiddle.c cause build failure as seen
below:

    Starting program: /home/npostavs/src/emacs/emacs-bootstrapping/src/temacs
	--batch --load loadup bootstrap
    [Thread debugging using libthread_db enabled]
    Using host libthread_db library "/usr/lib/libthread_db.so.1".
    Loading loadup.el (source)...
    Using load-path (/home/npostavs/src/emacs/emacs-bootstrapping/lisp
	/home/npostavs/src/emacs/emacs-bootstrapping/lisp/emacs-lisp
	/home/npostavs/src/emacs/emacs-bootstrapping/lisp/language
	/home/npostavs/src/emacs/emacs-bootstrapping/lisp/international
	/home/npostavs/src/emacs/emacs-bootstrapping/lisp/textmodes
	/home/npostavs/src/emacs/emacs-bootstrapping/lisp/vc)
    Loading emacs-lisp/byte-run (source)...
    Loading emacs-lisp/backquote (source)...
    Loading subr (source)...
    Loading version (source)...
    Loading widget (source)...
    Loading custom (source)...
    Loading emacs-lisp/map-ynp (source)...
    Loading international/mule (source)...
    Loading international/mule-conf (source)...

    lread.c:3914: Emacs fatal error: assertion failed: !NILP (Vpurify_flag)

    Breakpoint 1, terminate_due_to_signal at emacs.c:363
    363	  signal (sig, SIG_DFL);
    (gdb) bt
    #0  0x0000000000579826 in terminate_due_to_signal at emacs.c:363
    #1  0x000000000060ec33 in die at alloc.c:7352
    #2  0x000000000066db40 in intern_c_string_1 at lread.c:3914
    #3  0x0000000000576884 in intern_c_string at lisp.h:3790
    #4  0x00000000005dc84f in prepare_casing_context at casefiddle.c:69
    #5  0x00000000005dd37f in casify_object at casefiddle.c:311
    #6  0x00000000005dd47f in Fcapitalize at casefiddle.c:356
    #7  0x00000000006325ac in eval_sub at eval.c:2219
    #8  0x0000000000632368 in eval_sub at eval.c:2184
    #9  0x000000000063446c in apply_lambda at eval.c:2875
    #10 0x00000000006329af in eval_sub at eval.c:2294
    #11 0x000000000062d462 in Fprogn at eval.c:449
    #12 0x000000000062d4cf in prog_ignore at eval.c:461
    #13 0x000000000062f19c in Fwhile at eval.c:982
    #14 0x00000000006321f4 in eval_sub at eval.c:2172
    #15 0x000000000062d462 in Fprogn at eval.c:449
    #16 0x000000000062f0c4 in Flet at eval.c:963
    #17 0x00000000006321f4 in eval_sub at eval.c:2172
    #18 0x0000000000632963 in eval_sub at eval.c:2290
    #19 0x000000000062d462 in Fprogn at eval.c:449
    #20 0x000000000062f0c4 in Flet at eval.c:963
    #21 0x00000000006321f4 in eval_sub at eval.c:2172
    #22 0x0000000000668caa in readevalloop at lread.c:1927
    #23 0x0000000000667253 in Fload at lread.c:1332
    #24 0x0000000000632683 in eval_sub at eval.c:2233
    #25 0x0000000000668caa in readevalloop at lread.c:1927
    #26 0x0000000000667253 in Fload at lread.c:1332
    #27 0x0000000000632683 in eval_sub at eval.c:2233
    #28 0x0000000000631be5 in Feval at eval.c:2041
    #29 0x000000000057e1af in top_level_2 at keyboard.c:1121
    #30 0x000000000062ffc7 in internal_condition_case at eval.c:1324
    #31 0x000000000057e1f0 in top_level_1 at keyboard.c:1129
    #32 0x000000000062f51e in internal_catch at eval.c:1091
    #33 0x000000000057e0ea in command_loop at keyboard.c:1090
    #34 0x000000000057d6d5 in recursive_edit_1 at keyboard.c:697
    #35 0x000000000057d8b4 in Frecursive_edit at keyboard.c:768
    #36 0x000000000057b55b in main at emacs.c:1687

    Lisp Backtrace:
    "capitalize" (0xffffcf70)
    "format" (0xffffd130)
    "define-charset" (0xffffd370)
    "while" (0xffffd560)
    "let" (0xffffd7c0)
    "dolist" (0xffffd910)
    "let" (0xffffdb70)
    "load" (0xffffdfe0)
    "load" (0xffffe4a0)

* src/casefiddle.c (syms_of_casefiddle): Declare four new symbols:
Qtitlecase, Qspecial_uppercase, Qspecial_lowercase and
Qspecial_titlecase.
(prepare_casing_context): Use aforementioned symbols.

src/casefiddle.c

@@ -66,16 +66,16 @@ prepare_casing_context (struct casing_context *ctx,
   ctx->inword = false;
   ctx->titlecase_char_table
     = (flag < CASE_CAPITALIZE ? Qnil
-       : uniprop_table (intern_c_string ("titlecase")));
+       : uniprop_table (Qtitlecase));
   ctx->specialcase_char_tables[CASE_UP]
     = (flag == CASE_DOWN ? Qnil
-       : uniprop_table (intern_c_string ("special-uppercase")));
+       : uniprop_table (Qspecial_uppercase));
   ctx->specialcase_char_tables[CASE_DOWN]
     = (flag == CASE_UP ? Qnil
-       : uniprop_table (intern_c_string ("special-lowercase")));
+       : uniprop_table (Qspecial_lowercase));
   ctx->specialcase_char_tables[CASE_CAPITALIZE]
     = (flag < CASE_CAPITALIZE ? Qnil
-       : uniprop_table (intern_c_string ("special-titlecase")));
+       : uniprop_table (Qspecial_titlecase));
 
   /* If the case table is flagged as modified, rescan it.  */
   if (NILP (XCHAR_TABLE (BVAR (current_buffer, downcase_table))->extras[1]))
@@ -644,6 +644,11 @@ void
 syms_of_casefiddle (void)
 {
   DEFSYM (Qidentity, "identity");
+  DEFSYM (Qtitlecase, "titlecase");
+  DEFSYM (Qspecial_uppercase, "special-uppercase");
+  DEFSYM (Qspecial_lowercase, "special-lowercase");
+  DEFSYM (Qspecial_titlecase, "special-titlecase");
+
   defsubr (&Supcase);
   defsubr (&Sdowncase);
   defsubr (&Scapitalize);