Fix `eval_int` bug: crash on infinite bit sequence.
Fix `eval_int` bug: crash on infinite bit sequence.

Before:

    sawscript> eval_int {{ zero : [inf] }};
    saw: You have encountered a bug in Cryptol's implementation.
    *** Please create an issue at https://github.com/galoisinc/cryptol/issues

    %< ---------------------------------------------------
      Revision:  a048690e3aa233ec4d12608a761994b18dff1e33
      Branch:    master (non-committed files present during build)
      Location:  [Eval] fromVWord
      Message:   not a word
                 eval_int
    %< ---------------------------------------------------

After:

    sawscript> eval_int {{ zero : [inf] }};

    "eval_int" (<stdin>:1:1):
    eval_int: argument is not a finite bitvector
    sawscript>

GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script