* PGI was throwing the following error.
```
NVC++-S-0103-Illegal operand types for comparison operator (osc_rdma_frag.h: 75)
NVC++/power Linux 20.11-0: compilation completed with severe errors
```
* It must not have liked the inline declaration of the NULL pointer.
- So replace with a variable, as we do in other places in the code base.
Signed-off-by: Joshua Hursey <jhursey@us.ibm.com>
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