Prevent overflow in hash function str2tag_core() (yhirose#1529)

* str2tag_core(): prevent overflow

* Update httplib.h

works for all sizes of unsigned int and if there exists a #define for max

Author: JohannesFluegelHighyag

httplib.h

@@ -2978,9 +2978,13 @@ inline void get_remote_ip_and_port(socket_t sock, std::string &ip, int &port) {
 
 inline constexpr unsigned int str2tag_core(const char *s, size_t l,
                                            unsigned int h) {
-  return (l == 0) ? h
-                  : str2tag_core(s + 1, l - 1,
-                                 (h * 33) ^ static_cast<unsigned char>(*s));
+  return (l == 0)
+             ? h
+             : str2tag_core(
+                   s + 1, l - 1,
+                   //unsets the 6 high bits of h, therefore no overflow happens
+                   (((std::numeric_limits<unsigned int>::max)() >> 6) & h * 33) ^
+                       static_cast<unsigned char>(*s));
 }
 
 inline unsigned int str2tag(const std::string &s) {