Fix incorrect assert exposed by v63005/gtm8956 subtest

When ydb_chset env var is set to "M", compiling the following line

	set c=$PIECE("Hello "_$ZCH(190)_" world!",$ZCH(191),1,2)

Failed an assert

%YDB-F-ASSERT, Assert failed in sr_unix/gtm_utf8.c line 273 for expression (gtm_utf8_mode)

with the following C-stack

 #6  utf8_badchar_real () at sr_unix/gtm_utf8.c:273
 #7  utf8_badchar_dec () at sr_unix/gtm_utf8.c:249
 #8  valid_utf_string () at sr_unix/gtm_utf8.c:410
 #9  op_fnzpiece () at sr_port/op_fnzpiece.c:53
 #10 f_piece () at sr_unix/f_piece.c:171
 #11 expritem () at sr_port/expritem.c:619
 #12 expratom () at sr_port/expratom.c:29
 #13 eval_expr () at sr_port/eval_expr.c:63
 #14 expr () at sr_port/expr.c:29
 #15 m_write () at sr_port/m_write.c:71
 #16 cmd () at sr_port/cmd.c:302
 #17 linetail () at sr_port/linetail.c:35
 #18 line () at sr_port/line.c:230
 #19 compiler_startup () at sr_port/compiler_startup.c:144
 #20 compile_source_file () at sr_unix/source_file.c:132
 #21 gtm_compile () at sr_unix/gtm_compile.c:120

The assert that failed is correct. The issue is that we called the utf8_badchar_real() function
in non-UTF8 mode (i.e. when "gtm_utf8_mode" is 0). The issue is in op_fnzpiece() where we
invoke the valid_utf_string() function only if we are in UTF-8 mode (indicated by "gtm_utf8_mode == 1").
The assert (likely introduced as part of GTM-7762 in GT.M V6.3-000) is now fixed to take care of this.

sr_port/op_fnzpiece.c

@@ -3,6 +3,9 @@
  * Copyright (c) 2006-2015 Fidelity National Information 	*
  * Services, Inc. and/or its subsidiaries. All rights reserved.	*
  *								*
+ * Copyright (c) 2018 YottaDB LLC. and/or its subsidiaries.	*
+ * All rights reserved.						*
+ *								*
  *	This source code contains the intellectual property	*
  *	of its copyright holder(s), and is made available	*
  *	under a license.  If you do not know the terms of	*
@@ -17,6 +20,10 @@
 
 #include "gtm_utf8.h"
 
+#ifdef DEBUG
+GBLREF	boolean_t	gtm_utf8_mode;
+#endif
+
 /* --------------------------------------------------------------------
  * NOTE: This module is a near copy of sr_unix/op_fnpiece.c differing
  * only in that it calls "matchb" instead of "matchc" to do matching.
@@ -46,12 +53,10 @@ void op_fnzpiece(mval *src, mval *del, int first, int last, mval *dst)
 	char		*match_start;
 	int		match_res;
 	delimfmt	unichar;
-
 	DCL_THREADGBL_ACCESS;
-	SETUP_THREADGBL_ACCESS;
-
-	assert(!TREF(compile_time) || valid_utf_string(&src->str));
 
+	SETUP_THREADGBL_ACCESS;
+	assert(!TREF(compile_time) || !gtm_utf8_mode || valid_utf_string(&src->str));
 	if (--first < 0)
 		first = 0;
 	if ((piece_cnt = last - first) < 1)