busybox

Patch from Matt Kraai <kraai@alumni.carnegiemellon.edu>:

GNU tr complains on the following:

$ tr a ''
tr: when not truncating set1, string2 must be non-empty

BusyBox tr does not complain:

$ tr a ''
a
^D
0

It should result in an error, not in some spurious output.  The attached
patch generates an error.

Matt