allocated ppref data to zero in the case of an amap that has empty
space at the front.
Don't set anything in the ppref array if "len" is zero.
Many thanks to Sami Kantoluoto for providing gdb access to a machine
that would reliably crash with problems related to the above, and to
Stephan Thesing for corroborating that the patch properly addressed
the problem.
Note that the ar_pageoff (and related variables) types must be changed
soon. The use of "int" here is not theoretically sufficient.
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