RISC-V: fix memory reference to work for NULL pointer

While this is bit pathological case, some clients may use it to generate
segfault. SOMpp does this.

Signed-off-by: Jan Vrany <jan.vrany@fit.cvut.cz>

compiler/riscv/codegen/OMRMemoryReference.cpp

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2019, 2019 IBM Corp. and others
+ * Copyright (c) 2019, 2020 IBM Corp. and others
  *
  * This program and the accompanying materials are made available under
  * the terms of the Eclipse Public License 2.0 which accompanies this
@@ -178,13 +178,21 @@ void OMR::RV::MemoryReference::addToOffset(TR::Node *node, intptrj_t amount, TR:
 
    if (amount == 0)
       {
+      if (_baseRegister == NULL)
+         {
+         _baseRegister = cg->machine()->getRealRegister(TR::RealRegister::zero);
+         }
       return;
       }
 
    if (_baseRegister != NULL)
       {
       self()->consolidateRegisters(NULL, NULL, false, cg);
       }
+   else
+      {
+      _baseRegister = cg->machine()->getRealRegister(TR::RealRegister::zero);
+      }
 
    intptrj_t displacement = self()->getOffset() + amount;
    if (!VALID_ITYPE_IMM(displacement))
@@ -382,8 +390,6 @@ void OMR::RV::MemoryReference::consolidateRegisters(TR::Register *srcReg, TR::No
    {
    TR::Register *tempTargetRegister;
 
-   TR_ASSERT(false, "This method needs revision");
-
    if (self()->getUnresolvedSnippet() != NULL)
       {
       TR_UNIMPLEMENTED();