[libcxx] Fix a bug in strstreambuf::overflow.

The end pointer should point to one past the end of the newly allocated
buffer.

rdar://problem/24265174

Differential Revision: http://reviews.llvm.org/D20334


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@274132 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed