ExecutionEngine: Check for NULL ErrorStr before using it.

Patch by Yury Mikhaylov <yury.mikhaylov@gmail.com>.

llvm-svn: 156523

llvm/lib/ExecutionEngine/TargetSelect.cpp

@@ -56,6 +56,7 @@ TargetMachine *EngineBuilder::selectTarget(const Triple &TargetTriple,
     }
 
     if (!TheTarget) {
+      if (ErrorStr)
         *ErrorStr = "No available targets are compatible with this -march, "
                     "see -version for the available targets.\n";
       return 0;