Skip deletion of null framebuffers.

It's valid to have a map entry that is null to represent a framebuffer
that has been generated but never bound.  This was causing crashes after
dEQP pbo tests were run during dEQP shutdown.

BUG=angleproject:1149

Change-Id: Idcedac23e5c4e17ed025b10c2939572b394d3aec
Reviewed-on: https://chromium-review.googlesource.com/296962
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libANGLE/Context.cpp b/src/libANGLE/Context.cpp
index c131ef0..e5ab8a2 100644
--- a/src/libANGLE/Context.cpp
+++ b/src/libANGLE/Context.cpp
@@ -155,7 +155,7 @@
     for (auto framebuffer : mFramebufferMap)
     {
         // Default framebuffer are owned by their respective Surface
-        if (framebuffer.second->id() != 0)
+        if (framebuffer.second != nullptr && framebuffer.second->id() != 0)
         {
             SafeDelete(framebuffer.second);
         }