Fix copypaste error in glfwGetMonitorWorkarea

Related to #1322.
diff --git a/src/monitor.c b/src/monitor.c
index 869fdfb..e8d6bcb 100644
--- a/src/monitor.c
+++ b/src/monitor.c
@@ -341,8 +341,8 @@
         *ypos = 0;
     if (width)
         *width = 0;
-    if (width)
-        *width = 0;
+    if (height)
+        *height = 0;
 
     _GLFW_REQUIRE_INIT();