Fix a stack-smasher in PlatformMacOSX::GetSDKDirectory()

[aprantl]

Summary

GetSDKVersion expects the number of version fields not their byte size and will happily overwrite later contents of the stack.

I just found this by accident while stepping through the function and noticing that the backtrace disappears halfway through the function.

[jasonmolenda]

LGTM.

lldb/trunk/source/Plugins/Platform/MacOSX/PlatformMacOSX.cpp

ConstString PlatformMacOSX::GetSDKDirectory(lldb_private::Target &target) {
   ModuleSP exe_module_sp(target.GetExecutableModule());
   if (exe_module_sp) {
     ObjectFile *objfile = exe_module_sp->GetObjectFile();
     if (objfile) {
       std::string xcode_contents_path;
       std::string default_xcode_sdk;
       FileSpec fspec;
       uint32_t versions[2];
-      if (objfile->GetSDKVersion(versions, sizeof(versions))) {
+      if (objfile->GetSDKVersion(versions, 2)) {
         fspec = HostInfo::GetShlibDir();
         if (fspec) {
           std::string path;
           xcode_contents_path = fspec.GetPath();
           size_t pos = xcode_contents_path.find("/Xcode.app/Contents/");
           if (pos != std::string::npos) {
             // LLDB.framework is inside an Xcode app bundle, we can locate the
             // SDK from here