Fix a EnsureSufficientExecutionStack method for older versions of .NET

[Taritsyn]

Hello, Akash!

After upgrading to version 1.2.192, even running simple code on older versions of .NET leads to errors like this:

YantraJS.Core.JSException: RangeError: undefined
   at YantraJS.Core.Core.JSContextExtensions.EnsureSufficientExecutionStack(JSContext context)
   at YantraJS.Core.CallStackItem..ctor(JSContext context, String fileName, StringSpan& function, Int32 line, Int32 column)
   at body-:0,0(Closures , Arguments& )
   at YantraJS.Core.JSContext.Eval(String code, String codeFilePath)
   …

This PR fixes a error.

[ackava]

@Taritsyn Thanks for the fix.