const_cast: convert some C-style casts and remove some redundant ones

Change-Id: I74898e791e17971a3105febe660a2140aafaece9

hwpfilter/source/mzstring.h

@@ -88,7 +88,7 @@ class MzString
 
         int       length() const;
         const char*   c_str() const;
-        operator  char*()         { return (char *)c_str(); }
+        operator  char*()         { return const_cast<char *>(c_str()); }
 
 // If it is not possible to use the constructor with an initial
 // allocation size, use the following member to set the size.