[NFC] Check the nullness of pointer before dereference it in the assertion

This was part of #85050.

It is suggested to split the unrelated change as much as possible. So
here is the patch.

Author: ChuanqiXu9

clang/lib/Serialization/GeneratePCH.cpp

@@ -102,7 +102,7 @@ ReducedBMIGenerator::ReducedBMIGenerator(Preprocessor &PP,
 
 Module *ReducedBMIGenerator::getEmittingModule(ASTContext &Ctx) {
   Module *M = Ctx.getCurrentNamedModule();
-  assert(M->isNamedModuleUnit() &&
+  assert(M && M->isNamedModuleUnit() &&
          "ReducedBMIGenerator should only be used with C++20 Named modules.");
   return M;
 }