egg: Fix null termination behavior of egg_secure_strndup()

Even if the given string is shorter than n, the result should be null
terminated.  This matches the behavior of strndup().

egg/egg-secure-memory.c

@@ -1344,6 +1344,7 @@ egg_secure_strndup_full (const char *tag,
 	len = length + 1;
 	res = (char *)egg_secure_alloc_full (tag, len, options);
 	memcpy (res, str, len);
+	res[length] = '\0';
 	return res;
 }