scripts/functions: fix downloading with aria2

In case the remote file does not exist (and probably for some
other reasons as well), aria2 nonetheless creates an empty file
(or not empty for some other reasons).

The solution is to delete the file whenever aria2 fails.

 #!/bin/sh

 # Needed for systems without gettext

 $* -xc -o /dev/null - > /dev/null 2>&1 << EOF

 #include <libintl.h>

 int main()

 {

 	gettext("");

 	return 0;

 }

 EOF

 if [ ! "$?" -eq "0"  ]; then

 	echo -DKBUILD_NO_NLS;

 fi