[kernel] Check for nullptr after gfx_create_surface_from_display

If gfx_create_surface_from_display fails to allocate memory it'll
return nullptr. If this happens we're hosed, but it's better to fail
an assert than dereference a nullptr.

Test: booted Eve w/ gfxconsole.early, then "k gfx rgb_bars"
Change-Id: Ibdcfcf05800ab40a7cd37602877672a5688f4eef

kernel/lib/gfx/gfx.cpp

diff --git a/kernel/lib/gfx/gfx.cpp b/kernel/lib/gfx/gfx.cpp
index b4f29a0..16bd503 100644
--- a/kernel/lib/gfx/gfx.cpp
+++ b/kernel/lib/gfx/gfx.cpp
@@ -639,6 +639,7 @@
         return;
 
     gfx_surface* surface = gfx_create_surface_from_display(&info);
+    DEBUG_ASSERT(surface != nullptr);
 
     uint x, y;
     for (y = 0; y < surface->height; y++) {
@@ -667,6 +668,7 @@
         return;
 
     gfx_surface* surface = gfx_create_surface_from_display(&info);
+    DEBUG_ASSERT(surface != nullptr);
 
     uint x, y;
     for (y = 0; y < surface->height; y++) {
@@ -734,6 +736,7 @@
     }
 
     gfx_surface* surface = gfx_create_surface_from_display(&info);
+    DEBUG_ASSERT(surface != nullptr);
 
     if (!strcmp(argv[1].str, "display_info")) {
         printf("display:\n");