Get a dummy target to allow for calculator mode while processing backticks.

This also helps break the infinite loop caused when target is null.

So that we can have:

$ /Volumes/data/lldb/svn/trunk/build/Debug/lldb
(lldb) itob `0x123 - 0x321` 32 v
 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10  9  8  7  6  5  4  3  2  1  0
 [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 1, 0]
(lldb) 

llvm-svn: 143260