[lit] Fix UnboundLocalError for invalid shtest redirects

Replace the incorrect variable reference when invalid redirect is used.
This fixes the following issue:

  File "/usr/src/llvm/utils/lit/lit/TestRunner.py", line 316, in processRedirects
    raise InternalShellError(cmd, "Unsupported redirect: %r" % (r,))
UnboundLocalError: local variable 'r' referenced before assignment

which in turn broke shtest-shell.py and max-failures.py lit tests.

The breakage was introduced during refactoring in rL307310.

Differential Revision: https://reviews.llvm.org/D35857

llvm-svn: 309044