Fix potential buffer overflow in ssprintf

If vsnprintf fails for other reasons than too small a buffer, it returns
a negative value. Comparing that with a size_t value promotes the
negative value to unsigned, which makes for a very large result, almost
guaranteed to be larger than the buffer size.

src/lib/Standard.h

@@ -167,7 +167,9 @@ inline int ssprintf(char(&str)[N], const char *fmt, ...)
 {
 	va_list args; va_start(args, fmt);
 	int m = vsnprintf(str, N, fmt, args);
-	if (m >= N) { m = N-1; str[m] = 0; }
+	// Quick exit if vsnprintf failed
+	if (m < 0) return m;
+	if (static_cast<size_t>(m) >= N) { m = N-1; str[m] = 0; }
 	return m;
 }