Avoid deadlock when OpenGL context can't be created.

Previously, with aContext == nullptr, the mutex would lock, but not unlock.
Then it got deadlocked inside OPENGL_GAL dtor.

common/gal/opengl/opengl_gal.cpp

@@ -261,11 +261,17 @@ OPENGL_GAL::OPENGL_GAL( GAL_DISPLAY_OPTIONS& aDisplayOptions, wxWindow* aParent,
     {
         m_glMainContext = GL_CONTEXT_MANAGER::Get().CreateCtx( this );
 
+        if( !m_glMainContext )
+            throw std::runtime_error( "Could not create the main OpenGL context" );
+
         m_glPrivContext = m_glMainContext;
     }
     else
     {
         m_glPrivContext = GL_CONTEXT_MANAGER::Get().CreateCtx( this, m_glMainContext );
+
+        if( !m_glPrivContext )
+            throw std::runtime_error( "Could not create a private OpenGL context" );
     }
 
     m_shader = new SHADER();
@@ -2556,16 +2562,8 @@ void OPENGL_GAL::init()
 #endif /* wxCHECK_VERSION( 3, 0, 3 ) */
 
     // Check correct initialization from the constructor
-    if( !m_glMainContext )
-        throw std::runtime_error( "Could not create the main OpenGL context" );
-
-    if( !m_glPrivContext )
-        throw std::runtime_error( "Could not create a private OpenGL context" );
-
     if( m_tesselator == nullptr )
-        throw std::runtime_error( "Could not create the m_tesselator" );
-    // End initialization checks
-
+        throw std::runtime_error( "Could not create the tesselator" );
     GLenum err = glewInit();
 
     if( GLEW_OK != err )

common/gl_context_mgr.cpp

@@ -87,7 +87,8 @@ void GL_CONTEXT_MANAGER::DeleteAll()
 
 void GL_CONTEXT_MANAGER::LockCtx( wxGLContext* aContext, wxGLCanvas* aCanvas )
 {
-    wxCHECK( aCanvas || m_glContexts.count( aContext ) > 0, /* void */ );
+    assert( aContext );
+    wxCHECK( aContext && ( aCanvas || m_glContexts.count( aContext ) > 0 ), /* void */ );
 
     m_glCtxMutex.lock();
     wxGLCanvas* canvas = aCanvas ? aCanvas : m_glContexts.at( aContext );
@@ -106,7 +107,8 @@ void GL_CONTEXT_MANAGER::LockCtx( wxGLContext* aContext, wxGLCanvas* aCanvas )
 
 void GL_CONTEXT_MANAGER::UnlockCtx( wxGLContext* aContext )
 {
-    wxCHECK( m_glContexts.count( aContext ) > 0, /* void */ );
+    assert( aContext );
+    wxCHECK( aContext && m_glContexts.count( aContext ) > 0, /* void */ );
 
     if( m_glCtx == aContext )
     {