swresample/swresample: fix sample drop loop end condition

Fixes Ticket3985
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswresample/swresample.c

@@ -663,6 +663,8 @@ int swr_convert(struct SwrContext *s, uint8_t *out_arg[SWR_CH_MAX], int out_coun
         in_count = 0;
         if(ret>0) {
             s->drop_output -= ret;
+            if (!s->drop_output && !out_arg)
+                return 0;
             continue;
         }