C front-end: fix initialisation of large arrays

[tautschnig]

In 375e9a8 the construction of zero initialisers was switched to
array_of expressions for arrays with more than 1000 elements. This is
beneficial to avoid running out of memory, but didn't take into account
arrays with designated initialisers, which this patch fixes.

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[kroening]

Now tempted to always use the array_of, even for small arrays!