[csmith-project/csmith] e876ed: Fixes for defects reported by Pascal Cuoq.

[Xuejun Yang <jxyang@cs.utah.edu>]

  Branch: refs/heads/master
  Home:   https://github.com/csmith-project/csmith

  Commit: e876ed17ab2cd55f05659324f3b3560ee5bd42a2
      https://github.com/csmith-project/csmith/commit/e876ed17ab2cd55f05659324f3b3560ee5bd42a2
  Author: Xuejun Yang <jxyang@cs.utah.edu>
  Date:   2011-11-17 (Thu, 17 Nov 2011)

  Changed paths:
    M src/FactUnion.cpp
  M src/Lhs.cpp

  Log Message:
  -----------
  Fixes for defects reported by Pascal Cuoq.

Xuejun says: "Lhs.cpp is for fixing the first issue reported by Pascal;
FactUnion.cpp is for fixing the 2nd and 3rd issues.  I have run 100,000 test
cases for regression test."

`Lhs::make_random()': Handle certain cases in which unsigned bitfields are
undesirable.

`FactUnion::abstract_fact_for_assign()': If writing to an union field is
uncertain (due to dereference of a pointer which may points to a union
field or something else), mark the union as unreadable.