Move from strlen to strnlen_s.

Change-Id: I1d25a65e020b45b556723de14ef55a6140229dbb
diff --git a/src/std.c b/src/std.c
index fd52a0f..9f829e8 100644
--- a/src/std.c
+++ b/src/std.c
@@ -100,3 +100,18 @@
 fail:
 	panic("memmove_s failure");
 }
+
+size_t strnlen_s(const char *str, size_t strsz)
+{
+	const char *p = str;
+
+	if (str == NULL) {
+		return 0;
+	}
+
+	while (*p && strsz--) {
+		p++;
+	}
+
+	return p - str;
+}