zlib/ioapi.c: add 'return' to fseek64_file_func

This was causing savegame zip files to be "invalid" according to everything but ToME.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>

src/zlib/ioapi.c

@@ -180,6 +180,7 @@ static long ZCALLBACK fseek64_file_func (voidpf  opaque, voidpf stream, ZPOS64_T
     }
     ret = 0;
     PHYSFS_seek((PHYSFS_file *)stream, offset);
+    return ret;
 }