Fix error on windows:

c:\buildbot\lldb-windows-x86\lldb-windows-x86\llvm\tools\lldb\source\symbol\goastcontext.cpp(906) : warning C4715: 'lldb_private::GoASTContext::GetBitSize' : not all control paths return a value
c:\buildbot\lldb-windows-x86\lldb-windows-x86\llvm\tools\lldb\source\symbol\goastcontext.cpp(1175) : error C4716: 'lldb_private::GoASTContext::ConvertStringToFloatValue' : must return a value

llvm-svn: 247647