gcc 7 complains about possible format truncation:

mod_cdr_mongodb.c: In function 'my_on_reporting':
mod_cdr_mongodb.c:242:45: error: '%d' directive output may be truncated writing between 1 and 10 bytes into a region of size 4 [-Werror=format-truncation=]
   snprintf(idx_buffer, sizeof(idx_buffer), "%d", callflow_idx);

The char * idx_buffer has a size of 4 Bytes, and according to gcc's
calculation it is possible that up to 11 Bytes might be copied into it
via int bson_idx.

This commit adds an extra 8 Bytes to char * idx_buffer, which silences
the warnings.
Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>