Merge to XFA: Get rid of most instance of 'foo == NULL'

TBR=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1520063002 .

(cherry picked from commit e385244f8cd6ae376f6b3cf1265a0795d5d30eff)

Review URL: https://codereview.chromium.org/1528763003 .
diff --git a/fpdfsdk/src/fpdf_dataavail.cpp b/fpdfsdk/src/fpdf_dataavail.cpp
index 9fe2fa8..f4e235c 100644
--- a/fpdfsdk/src/fpdf_dataavail.cpp
+++ b/fpdfsdk/src/fpdf_dataavail.cpp
@@ -122,7 +122,7 @@
 
 DLLEXPORT FPDF_DOCUMENT STDCALL
 FPDFAvail_GetDocument(FPDF_AVAIL avail, FPDF_BYTESTRING password) {
-  if (avail == NULL)
+  if (!avail)
     return NULL;
   CPDF_Parser* pParser = new CPDF_Parser;
   pParser->SetPassword(password);