[CIR] fix deref nullptr when verify symbol for cir.get_global (#1143)

Author: HerrCai0907

clang/lib/CIR/Dialect/IR/CIRDialect.cpp

@@ -2023,8 +2023,8 @@ LogicalResult
 cir::GetGlobalOp::verifySymbolUses(SymbolTableCollection &symbolTable) {
   // Verify that the result type underlying pointer type matches the type of
   // the referenced cir.global or cir.func op.
-  auto op = symbolTable.lookupNearestSymbolFrom(*this, getNameAttr());
-  if (!(isa<GlobalOp>(op) || isa<FuncOp>(op)))
+  auto *op = symbolTable.lookupNearestSymbolFrom(*this, getNameAttr());
+  if (op == nullptr || !(isa<GlobalOp>(op) || isa<FuncOp>(op)))
     return emitOpError("'")
            << getName()
            << "' does not reference a valid cir.global or cir.func";

clang/test/CIR/IR/invalid.cir

@@ -1103,6 +1103,15 @@ module {
 
 // -----
 
+!s8i = !cir.int<s, 8>
+cir.func @no_reference_global() {
+  // expected-error @below {{'cir.get_global' op 'str' does not reference a valid cir.global or cir.func}}
+  %0 = cir.get_global @str : !cir.ptr<!s8i> 
+  cir.return 
+}
+
+// -----
+
 // expected-error@+1 {{invalid underlying type for long double}}
 cir.func @bad_long_double(%arg0 : !cir.long_double<!cir.float>) -> () {
   cir.return