[analyzer] Prevent crashing in NonNullParamChecker

[george.karpenkov]

Summary

Main property we care about is that we should only assume a location-like SVal.

[george.karpenkov]

https://bugs.llvm.org/show_bug.cgi?id=36381
rdar://37543426

lib/StaticAnalyzer/Checkers/NonNullParamChecker.cpp

for (unsigned idx = 0; idx < NumArgs; ++idx) {
 
     // If the value is unknown or undefined, we can't perform this check.
     const Expr *ArgE = Call.getArgExpr(idx);
     SVal V = Call.getArgSVal(idx);
     auto DV = V.getAs<DefinedSVal>();
     if (!DV)
       continue;
 
-    assert(!haveRefTypeParam || DV->getAs<Loc>());
-
     // Process the case when the argument is not a location.
     if (haveAttrNonNull && !DV->getAs<Loc>()) {
       // If the argument is a union type, we want to handle a potential
       // transparent_union GCC extension.
       if (!ArgE)
         continue;
 
       QualType T = ArgE->getType();
       const RecordType *UT = T->getAsUnionType();
       if (!UT || !UT->getDecl()->hasAttr<TransparentUnionAttr>())
         continue;
 
       auto CSV = DV->getAs<nonloc::CompoundVal>();
 
       // FIXME: Handle LazyCompoundVals?
       if (!CSV)
         continue;
 
       V = *(CSV->begin());
       DV = V.getAs<DefinedSVal>();
       assert(++CSV->begin() == CSV->end());
       // FIXME: Handle (some_union){ some_other_union_val }, which turns into
       // a LazyCompoundVal inside a CompoundVal.
-      if (!V.getAs<Loc>())
-        continue;
 
       // Retrieve the corresponding expression.
       if (const auto *CE = dyn_cast<CompoundLiteralExpr>(ArgE))
         if (const auto *IE = dyn_cast<InitListExpr>(CE->getInitializer()))
           ArgE = dyn_cast<Expr>(*(IE->begin()));
     }
 
+    if (!V.getAs<Loc>())
+      continue;
+
     ConstraintManager &CM = C.getConstraintManager();
     ProgramStateRef stateNotNull, stateNull;
     std::tie(stateNotNull, stateNull) = CM.assumeDual(state, *DV);
 
     // Generate an error node.  Check for a null node in case
     // we cache out.
     if (stateNull && !stateNotNull) {
       if (ExplodedNode *errorNode = C.generateErrorNode(stateNull)) {