[NFC]Add assert to avoid possibly deref nullptr (llvm#65564)

These 2 functions could be called by AsmPrinter::doInitialization in
AsmPrinter.cpp. doInitialization init MMI in the beginning`MMI = MMIWP ?
&MMIWP->getMMI() : nullptr;`, MMI has the possibility to be nullptr,
which could make the later deref crash. I think in most time MMI could
not be nullptr, but from the view of function implementation, it could
be, so I'd like to add assert to it, if this could be a problem, then we
could avoid crash.

Author: XinWang10

llvm/lib/CodeGen/AsmPrinter/AsmPrinterInlineAsm.cpp

@@ -400,6 +400,7 @@ void AsmPrinter::emitInlineAsm(const MachineInstr *MI) const {
         "Reserved registers on the clobber list may not be "
         "preserved across the asm statement, and clobbering them may "
         "lead to undefined behaviour.";
+    assert(MMI && "MMI can not be nullptr!");
     MMI->getModule()->getContext().diagnose(DiagnosticInfoInlineAsm(
         LocCookie, Msg, DiagnosticSeverity::DS_Warning));
     MMI->getModule()->getContext().diagnose(

llvm/lib/Target/X86/X86AsmPrinter.cpp

@@ -769,6 +769,7 @@ void X86AsmPrinter::emitStartOfAsmFile(Module &M) {
       if (!TT.isArch32Bit() && !TT.isArch64Bit())
         llvm_unreachable("CFProtection used on invalid architecture!");
       MCSection *Cur = OutStreamer->getCurrentSectionOnly();
+      assert(MMI && "MMI can not be nullptr!");
       MCSection *Nt = MMI->getContext().getELFSection(
           ".note.gnu.property", ELF::SHT_NOTE, ELF::SHF_ALLOC);
       OutStreamer->switchSection(Nt);