Driver: don't produce bogus osx version numbers if no version (aka 0) was specified.

llvm-svn: 84781
diff --git a/clang/lib/Driver/ToolChains.cpp b/clang/lib/Driver/ToolChains.cpp
index a0fefcd..398e5dd 100644
--- a/clang/lib/Driver/ToolChains.cpp
+++ b/clang/lib/Driver/ToolChains.cpp
@@ -38,7 +38,8 @@
   DarwinVersion[2] = _DarwinVersion[2];
 
   llvm::raw_string_ostream(MacosxVersionMin)
-    << "10." << DarwinVersion[0] - 4 << '.' << DarwinVersion[1];
+    << "10." << std::max(0, (int)DarwinVersion[0] - 4) << '.'
+    << DarwinVersion[1];
 
   // FIXME: Lift default up.
   IPhoneOSVersionMin = "3.0";