[APFloat] Swap an early out check so we do not dereference str.end().

Originally if D.firstSigDigit == str.end(), we will have already dereferenced
D.firstSigDigit in the first predicate.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185437 91177308-0d34-0410-b5e6-96231b3b80d8