avconv: initialize hwaccel_pix_fmt

Fixes an assertion failure in vdpau_get_buffer() when -hwaccel auto is
used, but hwaccel init fails.

avconv_opt.c

@@ -528,6 +528,7 @@ static void add_input_streams(OptionsContext *o, AVFormatContext *ic)
                 if (!ist->hwaccel_device)
                     exit_program(1);
             }
+            ist->hwaccel_pix_fmt = AV_PIX_FMT_NONE;
 
             break;
         case AVMEDIA_TYPE_AUDIO: