Do not parse sign if a sign is not the next legal character when parsing floating point from an input stream.  Fixes http://llvm.org/bugs/show_bug.cgi?id=11871

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@150609 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed