FramebufferGL: fix a crash when there is no color buffer.

BUG=chromium:675819

Change-Id: I7d143ba51bb16f28b145aa2b0410929d29e0b464
Reviewed-on: https://chromium-review.googlesource.com/422493
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/libANGLE/renderer/gl/FramebufferGL.cpp b/src/libANGLE/renderer/gl/FramebufferGL.cpp
index 9fde07c..3d0e900 100644
--- a/src/libANGLE/renderer/gl/FramebufferGL.cpp
+++ b/src/libANGLE/renderer/gl/FramebufferGL.cpp
@@ -289,7 +289,12 @@
     const Framebuffer *destFramebuffer       = context->getGLState().getDrawFramebuffer();
 
     const FramebufferAttachment *colorReadAttachment = sourceFramebuffer->getReadColorbuffer();
-    GLsizei readAttachmentSamples                    = colorReadAttachment->getSamples();
+
+    GLsizei readAttachmentSamples = 0;
+    if (colorReadAttachment != nullptr)
+    {
+        readAttachmentSamples = colorReadAttachment->getSamples();
+    }
 
     bool needManualColorBlit = false;