#1291 Fix segfault when handling error in part select of multidimensional array

[Henry Wong]

In PEIdent::elaborate_expr_net_bit_, the return value from elab_and_eval for the part select(?) isn't checked for failure. The example below segfaults because the part select [i] fails because i isn't defined.

Expected behaviour: It should fail elaboration without the segfault.

`module test();

reg [1:0][1:0] a;
reg b;
initial b=a[i];

endmodule`

(Patch also applies to 10.1.1)

[Stephen Williams]

Applied to git master.