Bill Fisher: This patch fixes a less likely case where '\b' can back up into invalid memory, when driven by a regex_iterator (for case 1, see r185273 or http://llvm.org/bugs/show_bug.cgi?id=16240)

The attached test program also supplies a test for the case 1 fix in r185273.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@186089 91177308-0d34-0410-b5e6-96231b3b80d8