dotgen: fix tracking of number of ranks during rank installation

The n and an members are intended to track the “number of nodes” and “allocated number of nodes”, respectively, in a rank. That makes this fix look straightforward, as it is simply aligning the values here with the next line performing the actual allocation. However cross-referencing this with another allocation site of ranking data in lib/dotgen/flat.c suggests there may be expected to be a +2 fudge factor here. Though the latter code lies in a function called abomination, so perhaps we should not take this as an educational usage.

I mention the above to explain that I am not sure this is the correct fix. But it addresses the problems of #1308 (closed) and does not break anything else in the test suite.

Gitlab: fixes #1308 (closed)
Reported-by: Google Autofuzz project