Implement cs_strnlen for Mac OS X portability (#2889)

Replace strnlen() call with a simple loop to find the bounded string
length. strnlen() is POSIX.1-2008 and not available on older platforms
like Mac OS X Leopard (10.5).

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

suite/cstest/include/helper.h

@@ -6,6 +6,8 @@
 
 #include <stddef.h>
 
+#include "../../../utils.h"
+
 #define MAX_ASM_TXT_MEM 1024
 #define X86_16 0
 #define X86_32 1

suite/cstest/src/helper.c

@@ -17,7 +17,7 @@ char *cs_strndup(const char *s, size_t n)
 	if (!s) {
 		return NULL;
 	}
-	size_t l = strnlen(s, n);
+	size_t l = cs_strnlen(s, n);
 	if (l == SIZE_MAX) {
 		return NULL;
 	}

utils.c

@@ -47,6 +47,21 @@ char *cs_strdup(const char *str)
 	return (char *)memmove(new, str, len);
 }
 
+// Portable strnlen replacement for platforms that lack it
+// (e.g. Mac OS X 10.5 Leopard).
+size_t cs_strnlen(const char *str, size_t n)
+{
+	if (!str)
+		return 0;
+
+	size_t l = 0;
+
+	while (l < n && str[l] != '\0')
+		l++;
+
+	return l;
+}
+
 // we need this since Windows doesn't have snprintf()
 int cs_snprintf(char *buffer, size_t size, const char *fmt, ...)
 {

utils.h

@@ -26,6 +26,11 @@ unsigned int count_positive8(const unsigned char *list);
 
 char *cs_strdup(const char *str);
 
+// Portable strnlen replacement. Returns the length of @str,
+// up to a maximum of @n. Needed because strnlen() is not
+// available on all platforms (e.g. Mac OS X 10.5 Leopard).
+size_t cs_strnlen(const char *str, size_t n);
+
 #define MIN(x, y) ((x) < (y) ? (x) : (y))
 
 // we need this since Windows doesn't have snprintf()