uchardet_get_charset() should never return NULL...

... to stay backward compatible with previous behavior.
About detection failure, our in-code documentation says:
"@return name of charset on success and "" on failure or pure ascii."
This behavior had been broken by commit 3a518c0, which returned NULL
instead. Our command-line tool was the first victim, segfaulting on
ASCII files.

src/uchardet.cpp

@@ -75,7 +75,7 @@ public:
 
     const char* GetCharset() const
     {
-        return m_charset;
+        return m_charset? m_charset : "";
     }
 };