Bugfix: error when compile with GCC

If one builds PDFium with 'is_clang = false' hence GCC is selected as
the compiler, an error of 'control reaches end of non-void function'
occurs. This commit fixes it.

Change-Id: I843c7d1ebb1a17ffbd6b419a1ed22772436726ef
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/51670
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/core/fpdfapi/render/cpdf_imagerenderer.cpp b/core/fpdfapi/render/cpdf_imagerenderer.cpp
index b4568ae..bc650ed 100644
--- a/core/fpdfapi/render/cpdf_imagerenderer.cpp
+++ b/core/fpdfapi/render/cpdf_imagerenderer.cpp
@@ -543,6 +543,8 @@
     case Mode::kTransform:
       return ContinueTransform(pPause);
   }
+  NOTREACHED();
+  return false;
 }
 
 bool CPDF_ImageRenderer::ContinueDefault(PauseIndicatorIface* pPause) {