OSHMEM: add compat to prev mxm version

fixed by ROman, reviewed by MikeD

Refs: #4672

This commit was SVN r31881.

oshmem/mca/atomic/mxm/atomic_mxm.h

@@ -64,12 +64,14 @@ OBJ_CLASS_DECLARATION(mca_atomic_mxm_module_t);
 
 END_C_DECLS
 
+#if MXM_API >= MXM_VERSION(2,0)
 static inline mxm_mem_key_t *to_mxm_mkey(sshmem_mkey_t *mkey) {
 
     if (0 == mkey->len) {
-        return &mxm_empty_mem_key;
+        return MXM_INVALID_MEM_HANDLE;
     }
     return (mxm_mem_key_t *)mkey->u.data;
 }
+#endif
 
 #endif /* MCA_ATOMIC_MXM_H */