Adjust conditions for some NOTREACHED() in CFX_DIBitmap.

In CompositeBitmap() and CompositeMask(), separate the conditionals for
NOTREACHED(). In one of the two conditions, remove the NOTREACHED(), as
pdf_scanlinecompositor_fuzzer can reach it.

Bug: chromium:1017703
Change-Id: I344d6987dc36864ed4a296c029f2d0e4d79e08de
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/75311
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

core/fxge/dib/cfx_dibitmap.cpp

diff --git a/core/fxge/dib/cfx_dibitmap.cpp b/core/fxge/dib/cfx_dibitmap.cpp
index f0b497f..bb5e517 100644
--- a/core/fxge/dib/cfx_dibitmap.cpp
+++ b/core/fxge/dib/cfx_dibitmap.cpp
@@ -878,14 +878,18 @@
                                    BlendMode blend_type,
                                    const CFX_ClipRgn* pClipRgn,
                                    bool bRgbByteOrder) {
-  if (!m_pBuffer)
-    return false;
-
-  if (pSrcBitmap->IsAlphaMask() || m_bpp < 8) {
+  if (pSrcBitmap->IsAlphaMask()) {
+    // Should have called CompositeMask().
     NOTREACHED();
     return false;
   }
 
+  if (!m_pBuffer)
+    return false;
+
+  if (m_bpp < 8)
+    return false;
+
   if (!GetOverlapRect(dest_left, dest_top, width, height,
                       pSrcBitmap->GetWidth(), pSrcBitmap->GetHeight(), src_left,
                       src_top, pClipRgn)) {
@@ -951,14 +955,18 @@
                                  BlendMode blend_type,
                                  const CFX_ClipRgn* pClipRgn,
                                  bool bRgbByteOrder) {
-  if (!m_pBuffer)
-    return false;
-
-  if (!pMask->IsAlphaMask() || m_bpp < 8) {
+  if (!pMask->IsAlphaMask()) {
+    // Should have called CompositeBitmap().
     NOTREACHED();
     return false;
   }
 
+  if (!m_pBuffer)
+    return false;
+
+  if (m_bpp < 8)
+    return false;
+
   if (!GetOverlapRect(dest_left, dest_top, width, height, pMask->GetWidth(),
                       pMask->GetHeight(), src_left, src_top, pClipRgn)) {
     return true;