The `ompi_comm_set` function never sets `NULL` to its first argument
`ncomm`. So `NULL` check is unnecessary in its callers. Furthermore,
`NULL` check may obscure a real return code when an error occurs
if the variable is initialized to a `NULL` value.
Also, `NULL` check is added in the `ompi_comm_set` function to
avoid segmentation fault in an out-of-memory condition.
Signed-off-by: KAWASHIMA Takahiro <t-kawashima@jp.fujitsu.com>
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