Fix corner case in quick-check generated tests

Thanks to Agata Murawska, a random seed that generates bad test cases
was found ("seed was 1768143307 2118231514, test size 74"). This is
due to the fact that some nodes were generated with tMem=0, fMem=0,
which means that pMem was NaN and thus the node was not comparing
equal to itself.

Worked around by limiting the test case generation to nodes with
positive values; a better fix would be to compare nodes differently
(subset of properties or ignore NaN values).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>