[Cabs2cil] avoid more crashes due to large array initializer

Note: trying to compile the initializer with gcc/CompCert likely results in
'no space left on device'. Clang 12 reports an unexpected warning
("initializer overrides prior initialization of this subobject"), and some
tests indicate it actually replaces index 72057594037927936 with 0, which would
explain why it compiles. Removing a digit from the index, for instance, results
in a similar issue as GCC/CompCert (except that Clang starts consuming all RAM
instead of simply disk space).