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

(cherry picked from commit fbfd7e65c8)

CONTRIBUTORS.md

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

README.md

@@ -124,6 +124,9 @@ information on what to include when reporting a bug.
 ## Changelog
 
  - Bugfix: Joysticks connected before init did not get gamepad mappings (#1996)
+ - [Win32] Bugfix: Content scale queries could fail silently (#1615)
+ - [Win32] Bugfix: Content scales could have garbage values if monitor was recently
+   disconnected (#1615)
  - [Cocoa] Bugfix: A dependency on an external constant caused crashes on macOS
    11 and earlier (#1985,#1994)
  - [X11] Bugfix: Icon pixel format conversion worked only by accident, relying on

src/win32_monitor.c

@@ -318,8 +318,19 @@ void _glfwGetMonitorContentScaleWin32(HMONITOR handle, float* xscale, float* ysc
 {
     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);