Win32: Fix bad content scale on monitor disconnect

The monitor handle could have become invalid just before the call to
GetDpiForMonitor.  It was possible for both window and monitor content
scale queries.

This ensures both that an appropriate error is emitted and that the
retrieved values are zero on error.

Fixes #1615

CONTRIBUTORS.md

@@ -62,6 +62,7 @@ video tutorials.
  - Mário Freitas
  - GeO4d
  - Marcus Geelnard
+ - ghuser404
  - Charles Giessen
  - Ryan C. Gordon
  - Stephen Gowen

README.md

@@ -202,6 +202,9 @@ information on what to include when reporting a bug.
    later (#1783,#1796)
  - [Win32] Bugfix: Compilation with LLVM for Windows failed (#1807,#1824,#1874)
  - [Win32] Bugfix: The foreground lock timeout was overridden, ignoring the user
+ - [Win32] Bugfix: Content scale queries could fail silently (#1615)
+ - [Win32] Bugfix: Content scales could have garbage values if monitor was recently
+   disconnected (#1615)
  - [Cocoa] Added support for `VK_EXT_metal_surface` (#1619)
  - [Cocoa] Added locating the Vulkan loader at runtime in an application bundle
  - [Cocoa] Moved main menu creation to GLFW initialization time (#1649)

src/win32_monitor.c

@@ -317,8 +317,19 @@ void _glfwGetHMONITORContentScaleWin32(HMONITOR handle, float* xscale, float* ys
 {
     UINT xdpi, ydpi;
 
+    if (xscale)
+        *xscale = 0.f;
+    if (yscale)
+        *yscale = 0.f;
+
     if (IsWindows8Point1OrGreater())
-        GetDpiForMonitor(handle, MDT_EFFECTIVE_DPI, &xdpi, &ydpi);
+    {
+        if (GetDpiForMonitor(handle, MDT_EFFECTIVE_DPI, &xdpi, &ydpi) != S_OK)
+        {
+            _glfwInputError(GLFW_PLATFORM_ERROR, "Win32: Failed to query monitor DPI");
+            return;
+        }
+    }
     else
     {
         const HDC dc = GetDC(NULL);