[Swan-dev] replace type-safe min() with PMIN() in the realloc code

[D. Hugh Redelmeier]

| commit 94c2c3708a98ffde7cf0f14f35689c8a9816eed1
| Author: Andrew Cagney <cagney at gnu.org>
| Date:   Wed Apr 17 17:00:11 2019 -0400
| 
|     libswan: replace type-safe min() with PMIN() in the realloc code
|     
|     Don't assume that 'unsigned long' and size_t are type compatible.
|     
|     Spotted by Tuomo.

Why is union mhdr's i.size not of type size_t?

Would making it so not be a cleaner change?

(I may well have been the one who created it in the first place.  If so, 
I don't remember why I didn't use size_t.)