Infinite loop for truncated signature or delta files.

This might be related to #29
I was investigating why delta files seem to have additional characters on the end when generating deltas of identical files...

With the current rdiff,

echo "Hello world" > input
rdiff signature input sig
rdiff delta sig input delt
rdiff patch input delt output
# all is fine
# time to hang the process, truncate the delta file
dd if=delt of=delt2 bs=1 count=1
rdiff patch input delt2 output
# HANG, infinite loop somewhere
# Note that when the delta file is big enough, THEN the problem is handled properly
dd if=delt of=delt2 bs=1 count=4
rdiff patch input delt2 output
librsync: ERROR: (librsync) patch job failed: unexpected end of input
librsync: ERROR: unexpected end of input