[MC] Replace one nullable getCurrentSectionOnly with CurFrag

We will implement getCurrentSectionOnly with `CurFrag->getParent()`,
which is non-null. Eliminate a nullable use.

Author: MaskRay

llvm/lib/MC/MCELFStreamer.cpp

@@ -628,8 +628,8 @@ void MCELFStreamer::finishImpl() {
   }
 
   // Ensure the last section gets aligned if necessary.
-  MCSection *CurSection = getCurrentSectionOnly();
-  setSectionAlignmentForBundling(getAssembler(), CurSection);
+  if (MCFragment *F = getCurrentFragment())
+    setSectionAlignmentForBundling(getAssembler(), F->getParent());
 
   finalizeCGProfile();
   emitFrames(nullptr);

llvm/lib/MC/MCObjectStreamer.cpp

@@ -143,7 +143,8 @@ void MCObjectStreamer::emitFrames(MCAsmBackend *MAB) {
 }
 
 MCFragment *MCObjectStreamer::getCurrentFragment() const {
-  assert(CurFrag->getParent() == getCurrentSection().first);
+  assert(!getCurrentSection().first ||
+         CurFrag->getParent() == getCurrentSection().first);
   return CurFrag;
 }