make puff const correct instead of const-casting

include/libtorrent/puff.hpp

@@ -29,5 +29,5 @@
 
 int puff(unsigned char *dest,           /* pointer to destination pointer */
          boost::uint32_t *destlen,        /* amount of output space */
-         unsigned char *source,         /* pointer to source data pointer */
+         const unsigned char *source,         /* pointer to source data pointer */
          boost::uint32_t *sourcelen);     /* amount of input available */

src/gzip.cpp

@@ -220,8 +220,7 @@ namespace libtorrent
 			}
 
 			ret = puff(reinterpret_cast<unsigned char*>(&buffer[0]), &destlen
-				, const_cast<unsigned char*>(
-					reinterpret_cast<const unsigned char*>(in)), &srclen);
+				, reinterpret_cast<const unsigned char*>(in), &srclen);
 
 			// if the destination buffer wasn't large enough, double its
 			// size and try again. Unless it's already at its max, in which

src/puff.cpp

@@ -99,7 +99,7 @@ struct state {
     boost::uint32_t outcnt;       /* bytes written to out so far */
 
     /* input state */
-    unsigned char *in;          /* input buffer */
+    const unsigned char *in;          /* input buffer */
     boost::uint32_t inlen;        /* available input at in */
     boost::uint32_t incnt;        /* bytes read so far */
     int bitbuf;                 /* bit buffer */
@@ -748,7 +748,7 @@ local int dynamic(struct state *s)
  */
 int puff(unsigned char *dest,           /* pointer to destination pointer */
          boost::uint32_t *destlen,        /* amount of output space */
-         unsigned char *source,         /* pointer to source data pointer */
+         const unsigned char *source,         /* pointer to source data pointer */
          boost::uint32_t *sourcelen)      /* amount of input available */
 {
     struct state s;             /* input/output state */