fate-run: make avconv() properly deal with multiple inputs.

tests/fate-run.sh

@@ -75,7 +75,13 @@ probefmt(){
 }
 
 avconv(){
-    run avconv -nostats -threads $threads -thread_type $thread_type -cpuflags $cpuflags "$@"
+    dec_opts="-threads $threads -thread_type $thread_type"
+    avconv_args="-nostats -cpuflags $cpuflags"
+    for arg in $@; do
+        [ ${arg} = -i ] && avconv_args="${avconv_args} ${dec_opts}"
+        avconv_args="${avconv_args} ${arg}"
+    done
+    run avconv ${avconv_args}
 }
 
 framecrc(){