Fix glfw crashes when passing null for optional output arguments of some functions

[doanamo]

Do not write output values when optional output pointer is null for calls of following functions:

• glfwGetFramebufferSize (width/height)
• glfwGetWindowSize (width/height)
• glfwGetWindowPos (x/y)

This is in pair with native GLFW 3.2 implementation and documentation for these functions.

Example crash callstack and my code that ran into it:

RuntimeError: abort(segmentation fault storing 4 bytes to address 0) at jsStackTrace@http://localhost:8000/Example.js:2219:17
stackTrace@http://localhost:8000/Example.js:2236:16
abort@http://localhost:8000/Example.js:1995:44
SAFE_HEAP_STORE@http://localhost:8000/Example.js:931:23
setValue@http://localhost:8000/Example.js:875:34
_glfwGetFramebufferSize@http://localhost:8000/Example.js:9572:17
System::Window::GetWidth() const@http://localhost:8000/Example.js line 2133 > WebAssembly.instantiate:wasm-function[12079]:0x1c19c7

int Window::GetWidth() const
{
    int width = 0;
    glfwGetFramebufferSize(m_handle, &width, nullptr);
    return width;
}

int Window::GetHeight() const
{
    int height = 0;
    glfwGetFramebufferSize(m_handle, nullptr, &height);
    return height;
}

Thank you!

[welcome]

Thank you for submitting a pull request! If this is your first PR, make sure to add yourself to AUTHORS.

[kripken]

Thanks! Looks good, just some whitespace changes needed before landing.

[kripken]

space after if please.

[doanamo]

Ops! Corrected and updated. :)

[kripken]

Perfect, thanks @gunstarpl !