We need to first clean the symtable and then check whether a cache
slot is available for it. Otherwise, it may happen that a destructor
runs while cleaning the table and uses up all the remaining slots
in the cache.
This is particularly insidious because once we overflow the cache,
the first pointer we modify is symtable_cache_ptr, making it hard
to understand what happened after the fact.
Fixes oss-fuzz #30815.
