BUG/MINOR: ssl: Do not free garbage pointers on memory allocation failure

BUG/MINOR: ssl: Do not free garbage pointers on memory allocation failure

In `ckch_inst_sni_ctx_to_sni_filters` use `calloc()` to allocate the filter
array. When the function fails to allocate memory for a single entry the
whole array will be `free()`d using free_sni_filters(). With the previous
`malloc()` the pointers for entries after the failing allocation could
possibly be a garbage value.

This bug was introduced in commit 38df1c8006a2adf97f4ad5a183f80cfdcba3da8a,
which is 2.2+. No backport needed.