Use av_strlcpy instead of snprintf without formatting.

Prevents warnings about "format not a string literal and no format arguments".
patch by Patrik Kullman, patrik yes nu

Originally committed as revision 17141 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffmpeg.c

@@ -3717,7 +3717,7 @@ static int opt_preset(const char *opt, const char *arg)
     }
     if(!f && ((arg[0]=='.' && arg[1]=='/') || arg[0]=='/' ||
               is_dos_path(arg))){
-        snprintf(filename, sizeof(filename), arg);
+        av_strlcpy(filename, arg, sizeof(filename));
         f= fopen(filename, "r");
     }