#2977 Fast arrays return "false" for non-existing items

[David Scherfgen]

Fast arrays return false for non-existing items. This behavior differs from that of non-fast arrays and isn't documented anywhere, as far as I can see.

(%i1) f[42] : 1$
(%i2) f[42];
(%o2) 1
(%i3) f[43];
(%o3) f[43]
(%i4) use_fast_arrays : true$
(%i5) g[42] : 1$
(%i6) g[42];
(%o6) 1
(%i7) g[43];
(%o7) false