stream: allow invalid handle detection in cached_file

Otherwise one cannot distinguish between file open but in failed state
and file not open.

Signed-off-by: Simon Rozman <simon@rozman.si>

include/stdex/stream.hpp

@@ -3167,6 +3167,11 @@ namespace stdex
 				open(filename.c_str(), mode);
 			}
 
+			///
+			/// Returns true if file has a valid handle
+			///
+			inline operator bool() const noexcept { return m_source; }
+
 		protected:
 			file m_source;
 		};