[Flang] Fix usage of uninitialized resolution variable

[pscoro]

Summary

Recent Flang PowerPC intrinsics patch used resolution variable without checking if it exists first, causing segmentation faults in some scenarios. This patch checks that the resolution variable exists first before usage.

flang/lib/Semantics/expression.cpp

auto ExpressionAnalyzer::GetCalleeAndArguments(const parser::Name &name,
   bool isExplicitIntrinsic{ultimate.attrs().test(semantics::Attr::INTRINSIC)};
   const Symbol *resolution{nullptr};
   if (isGenericInterface || isExplicitIntrinsic) {
     ExpressionAnalyzer::AdjustActuals noAdjustment;
     auto pair{ResolveGeneric(*symbol, arguments, noAdjustment, isSubroutine,
         mightBeStructureConstructor)};
     resolution = pair.first;
     dueToAmbiguity = pair.second;
+    if (resolution) {
       if (context_.GetPPCBuiltinsScope() &&
           resolution->name().ToString().rfind("__ppc_", 0) == 0) {
         semantics::CheckPPCIntrinsic(
             *symbol, *resolution, arguments, GetFoldingContext());
       }
-    if (resolution) {
       // re-resolve name to the specific procedure
       name.symbol = const_cast<Symbol *>(resolution);
     }
   } else if (IsProcedure(ultimate) &&
       ultimate.attrs().test(semantics::Attr::ABSTRACT)) {
     Say("Abstract procedure interface '%s' may not be referenced"_err_en_US,
         name.source);
   } else {