[igraph] iterator selections and logic short-circuiting

[Carl Pearson]

Using the iterator function, with a logical query

E(g)[!a && b]

is causing complaints when '!a' evaluates FALSE (i.e., the expression
should be short-circuited) and 'b' is null.  I'm working around this
currently, but this struck me as indicating an opportunity to trim
some unnecessary operations under the iterator hood (no clue how it
actually works, however, so can't suggest anything).

Also, provided there isn't some gross performance trade, clearly I'd
like to use the iterator query this way.

C