defMAC construction up a scope to make it live long enough. (#54702)

Fixes #54649

src/coreclr/jit/morph.cpp

@@ -6046,6 +6046,10 @@ GenTree* Compiler::fgMorphField(GenTree* tree, MorphAddrContext* mac)
  }
 #endif
 
+ // Create a default MorphAddrContext early so it doesn't go out of scope
+ // before it is used.
+ MorphAddrContext defMAC(MACK_Ind);
+
  /* Is this an instance data member? */
 
  if (objRef)
@@ -6136,7 +6140,6 @@ GenTree* Compiler::fgMorphField(GenTree* tree, MorphAddrContext* mac)
 
  // NULL mac means we encounter the GT_FIELD first. This denotes a dereference of the field,
  // and thus is equivalent to a MACK_Ind with zero offset.
- MorphAddrContext defMAC(MACK_Ind);
  if (mac == nullptr)
  {
  mac = &defMAC;