- [SF 2513643] Seg fault in Poco::UTF8::toLower on 64-bit Linux

- removed support for 5- and 6-byte sequences
- fixed error counting in StreamConverterBuf::readFromDevice()
- added std::dec to poco_stdout_dbg and poco_stderr_dbg macros

Foundation/include/Poco/UTF8Encoding.h

@@ -59,6 +59,16 @@ public:
 	int convert(const unsigned char* bytes) const;
 	int convert(int ch, unsigned char* bytes, int length) const;
 	
+	static bool isLegal(const unsigned char *bytes, int length);
+		/// Utility routine to tell whether a sequence of bytes is legal UTF-8.
+		/// This must be called with the length pre-determined by the first byte.
+		/// The sequence is illegal right away if there aren't enough bytes 
+		/// available. If presented with a length > 4, this function returns false.
+		/// The Unicode definition of UTF-8 goes up to 4-byte sequences.
+		/// 
+		/// Adapted from ftp://ftp.unicode.org/Public/PROGRAMS/CVTUTF/ConvertUTF.c
+		/// Copyright 2001-2004 Unicode, Inc.
+
 private:
 	static const char* _names[];
 	static const CharacterMap _charMap;