Never pass a signed char to ctype macros like isdigit()

Bug: None
Change-Id: I451bb2c1f175a77aefbc8363009bf35a769fe941
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/264442
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#37037}

pc/ice_server_parsing.cc

@@ -92,7 +92,7 @@ static bool GetServiceTypeAndHostnameFromUri(const std::string& in_str,
 static bool ParsePort(const std::string& in_str, int* port) {
   // Make sure port only contains digits. FromString doesn't check this.
   for (const char& c : in_str) {
-    if (!std::isdigit(c)) {
+    if (!std::isdigit(static_cast<unsigned char>(c))) {
       return false;
     }
   }