As a result of this change, the basic_stringbuf constructor that
takes a mode ends up leaving __hm_ set to 0, causing the comparison
"__hm_ - __str_.data() < __noff" in seekoff() to succeed, which caused
the function to incorrectly return -1. The fix is to account for the
possibility of __hm_ being 0 when computing the distance from __hm_
to the start of the string.
Differential Revision: https://reviews.llvm.org/D41319
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@321124 91177308-0d34-0410-b5e6-96231b3b80d8
