Use swapchain's m_num only when attachment array has valid content

src/renderer_mtl.mm

@@ -1465,7 +1465,7 @@ namespace bgfx { namespace mtl
 
 			uint32_t numMrt = 1;
 			FrameBufferHandle fbh = m_fbh;
-			if (isValid(fbh) )
+			if (isValid(fbh) && m_frameBuffers[fbh.idx].m_swapChain == NULL)
 			{
 				const FrameBufferMtl& fb = m_frameBuffers[fbh.idx];
 				numMrt = bx::uint32_max(1, fb.m_num);