ompi/oshmem: fix shmem_free to perform no-op on null ptr

Signed-off-by: Scott Miller <scott.miller1@ibm.com>

oshmem/shmem/c/shmem_free.c

@@ -1,6 +1,7 @@
 /*
  * Copyright (c) 2013-2015 Mellanox Technologies, Inc.
  *                         All rights reserved.
+ * Copyright (c) 2018      IBM Corporation.  All rights reserved.
  * $COPYRIGHT$
  *
  * Additional copyrights may follow
@@ -41,7 +42,12 @@ static inline void _shfree(void* ptr)
 {
     int rc;
 
-    RUNTIME_CHECK_INIT(); RUNTIME_CHECK_ADDR(ptr);
+    RUNTIME_CHECK_INIT();
+    if (NULL == ptr) {
+        return;
+    }
+
+    RUNTIME_CHECK_ADDR(ptr);
 
 #if OSHMEM_SPEC_COMPAT == 1
     shmem_barrier_all();