Suppress compiler warnings about useless comparison of unsigned to zero.

Reportedly, some compilers warn about tests like "c < 0" if c is unsigned,
and hence complain about the character range checks I added in commit
3bb3f42f.  This is a bit of a pain since
the regex library doesn't really want to assume that chr is unsigned.
However, since any such reconfiguration would involve manual edits of
regcustom.h anyway, we can put it on the shoulders of whoever wants to
do that to adjust this new range-checking macro correctly.

Per gripes from Coverity and Andres.

src/backend/regex/regc_lex.c

@@ -813,13 +813,13 @@ lexescape(struct vars * v)
 			break;
 		case CHR('u'):
 			c = lexdigits(v, 16, 4, 4);
-			if (ISERR() || c < CHR_MIN || c > CHR_MAX)
+			if (ISERR() || !CHR_IS_IN_RANGE(c))
 				FAILW(REG_EESCAPE);
 			RETV(PLAIN, c);
 			break;
 		case CHR('U'):
 			c = lexdigits(v, 16, 8, 8);
-			if (ISERR() || c < CHR_MIN || c > CHR_MAX)
+			if (ISERR() || !CHR_IS_IN_RANGE(c))
 				FAILW(REG_EESCAPE);
 			RETV(PLAIN, c);
 			break;
@@ -837,7 +837,7 @@ lexescape(struct vars * v)
 		case CHR('x'):
 			NOTE(REG_UUNPORT);
 			c = lexdigits(v, 16, 1, 255);		/* REs >255 long outside spec */
-			if (ISERR() || c < CHR_MIN || c > CHR_MAX)
+			if (ISERR() || !CHR_IS_IN_RANGE(c))
 				FAILW(REG_EESCAPE);
 			RETV(PLAIN, c);
 			break;

src/include/regex/regcustom.h

@@ -68,6 +68,17 @@ typedef int celt;				/* type to hold chr, or NOCELT */
 #define CHR_MAX 0x7ffffffe		/* CHR_MAX-CHR_MIN+1 must fit in an int, and
 								 * CHR_MAX+1 must fit in both chr and celt */
 
+/*
+ * Check if a chr value is in range.  Ideally we'd just write this as
+ *		((c) >= CHR_MIN && (c) <= CHR_MAX)
+ * However, if chr is unsigned and CHR_MIN is zero, the first part of that
+ * is a no-op, and certain overly-nannyish compilers give warnings about it.
+ * So we leave that out here.  If you want to make chr signed and/or CHR_MIN
+ * not zero, redefine this macro as above.  Callers should assume that the
+ * macro may multiply evaluate its argument, even though it does not today.
+ */
+#define CHR_IS_IN_RANGE(c)	((c) <= CHR_MAX)
+
 /* functions operating on chr */
 #define iscalnum(x) pg_wc_isalnum(x)
 #define iscalpha(x) pg_wc_isalpha(x)