Fix to crash condition in amd

[goulart-paul]

Fixes a condition causing an assertion failure in the AMD ordering. The issue arises when (pme1,pme2) = (0,-1), which should generate an empty range for pme1..(pme2+1), but was instead causing "add with overflow" errors.

Example that triggers this error condition is here: https://pastebin.com/qdGq83kF