Treat paths with embedded NUL bytes as invalid (#1765)

Fixes #1763.

httplib.h

@@ -2413,6 +2413,7 @@ inline bool is_valid_path(const std::string &path) {
     // Read component
     auto beg = i;
     while (i < path.size() && path[i] != '/') {
+      if (path[i] == '\0') { return false; }
       i++;
     }