LibGL: Support missing context in glGetError and glGetIntegerv

In its current state, ScummVM seems to invoke these methods just after
destroying the current GL context. According to the OpenGL spec:

    "Issuing GL commands when the program does not have a current
     context results in undefined behavior, up to and including program
     termination."

Our old behavior was to deref a `nullptr`, which isn't that great. For
now, protect these two methods. If other ports seem to misbehave as
well, we can always expand the check to other methods.

Userland/Libraries/LibGL/GLContext.cpp

@@ -5,6 +5,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include "GLContext.h"
 #include "SoftwareGLContext.h"
 #include <LibGfx/Bitmap.h>
 

Userland/Libraries/LibGL/GLContext.h

@@ -14,6 +14,16 @@
 
 namespace GL {
 
+#define VERIFY_CURRENT_CONTEXT() \
+    if (!g_gl_context) {         \
+        return;                  \
+    }
+
+#define VERIFY_CURRENT_CONTEXT_OR_VALUE(value) \
+    if (!g_gl_context) {                       \
+        return value;                          \
+    }
+
 class GLContext {
 public:
     virtual ~GLContext();

Userland/Libraries/LibGL/GLUtils.cpp

@@ -67,6 +67,7 @@ void glViewport(GLint x, GLint y, GLsizei width, GLsizei height)
 
 GLenum glGetError()
 {
+    VERIFY_CURRENT_CONTEXT_OR_VALUE(GL_NONE);
     return g_gl_context->gl_get_error();
 }
 
@@ -117,6 +118,7 @@ void glGetFloatv(GLenum pname, GLfloat* params)
 
 void glGetIntegerv(GLenum pname, GLint* data)
 {
+    VERIFY_CURRENT_CONTEXT();
     g_gl_context->gl_get_integerv(pname, data);
 }