Handle degenerate case of nMapNum == 0
Handle degenerate case of nMapNum == 0

...appears to be occasionally necessary, probably since
d8e7bc042bd0912a66c231b101cecda0f838a80f "Remove hack of replacing 0 with
1/LONG_MAX"

Change-Id: I1eb7277bfdc9722c761bd03678cd770a3763de1f

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core