[Clang] Prevent null pointer dereference in DiagnoseDeclAvailability() (llvm#97095)

This patch adds an assertion in the DiagnoseDeclAvailabilit() function
to ensure that the expected availability attribute is not null before
they are passed to hasMatchingEnvironmentOrNone() to prevent potential
null pointer dereferences and improve the robustness of the availability
diagnostics process.

Author: smanna12

clang/lib/Sema/SemaAvailability.cpp

@@ -842,6 +842,7 @@ void DiagnoseUnguardedAvailability::DiagnoseDeclAvailability(
 
     const AvailabilityAttr *AA =
       getAttrForPlatform(SemaRef.getASTContext(), OffendingDecl);
+    assert(AA != nullptr && "expecting valid availability attribute");
     bool EnvironmentMatchesOrNone =
         hasMatchingEnvironmentOrNone(SemaRef.getASTContext(), AA);
     VersionTuple Introduced = AA->getIntroduced();