#2085 `~at` with a spread array whose leading slot is empty segfaults: `validateIndex` takes `items()` alongside `data()`

[Moritz Hoffmann]

a = (1,2)
say a~at((,2))

exits with SIGSEGV, rc 139. Measured deterministic, three runs of three, and the same for
a~at((,,3)).

ArrayClass::validateIndex spreads a lone array argument into the subscript list by taking its item
count alongside its slot array (interpreter/classes/ArrayClass.cpp:1219-1226):
indexCount = indirect->items() and index = indirect->data(). But items() counts the non-empty
slots while data() returns the raw slot array, so an array whose leading slot is empty and which
holds exactly one item hands validateSingleDimensionIndex an indexCount of 1 with
index[0] == OREF_NULL. Line 1264 then calls index[0]->requiredPositive(argPosition) on that null.

The neighbours are all clean and identify the shape exactly. a~at((1,)) answers 1 (one item,
leading slot filled); a~at((1,,3)) raises 93.926 (two items, so the count is rejected before either
subscript is read); a~at((,)) raises 93.901 (no items at all). The crashing shape is any array whose
first filled slot is not slot one and which holds exactly one item.