Return Utf8DecodeError when passed an incomplete code point

Also ensure that len set in all code paths, otherwise it leads to a call to remove_prefix with garbage.

Source/utils/utf8.cpp

@@ -21,7 +21,8 @@ char32_t DecodeFirstUtf8CodePoint(string_view input, uint8_t *len)
 			return Utf8DecodeError;
 		}
 	}
-	return codepoint;
+	*len = input.size();
+	return Utf8DecodeError;
 }
 
 } // namespace devilution