Refuse a ':' in a path a mount point serves on Windows (#2643)

On NTFS a ':' in a path names a stream of the file, so /dir/test.html::$DATA opened the default stream of dir/test.html and passed a set_pre_routing_handler() guard on /dir/test.html, and /setup.exe:Zone.Identifier served the stream Windows attaches to downloaded files. is_valid_path() now refuses a ':' on Windows.

Elsewhere a ':' is an ordinary file name character, so it is still served: a file such as 2024-01-01T10:00:00.log would otherwise answer 404. The test pins both sides.

httplib.h

@@ -6007,9 +6007,10 @@ inline bool is_valid_close_payload(const std::string &payload) {
 namespace detail {
 
 // A path a mount point serves: absolute, with no empty segment except a
-// trailing '/' and no segment ending in '.' or a space (which covers "." and
-// ".." and names Windows trims into another file's name), so the file served
-// is the one that req.path names to the handlers that ran before it.
+// trailing '/', no segment ending in '.' or a space (which covers "." and
+// ".." and names Windows trims into another file's name) and, on Windows, no
+// ':' (which names an NTFS stream, so "a.txt::$DATA" is a.txt), so the file
+// served is the one that req.path names to the handlers that ran before it.
 inline bool is_valid_path(const std::string &path) {
   if (path.empty() || path[0] != '/') { return false; }
 
@@ -6017,6 +6018,9 @@ inline bool is_valid_path(const std::string &path) {
   for (auto i = beg; i <= path.size(); i++) {
     if (i < path.size() && path[i] != '/') {
       if (path[i] == '\0' || path[i] == '\\') { return false; }
+#ifdef _WIN32
+      if (path[i] == ':') { return false; }
+#endif
       continue;
     }
     auto len = i - beg;