sql: fix error handling in sql_analysis_load()

Previously if an error occured in box_index_len() called from
sql_analysis_load(), the return code (-1 on error) was cast to uint32_t
and used later as size of memory to be allocated. This lead to assertion
failures in slab_order() since allocation size was too big. This was
discovered during investigation of #3779.
Fix error handling and add some error logging.

Follow-up #3779