Fix leak of symbol name in block_symbol_cache

Fix leak of symbol name in block_symbol_cache

A symbol not found inserted in the cache has a xstrdup-ed name
that must be freed, but only the struct block_symbol_cache is freed.
Add a function destroy_block_symbol_cache that clears all slots
before releasing the cache.

2019-12-03  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
	* symtab.c (symbol_cache_clear_slot):  Move close to cleared type.
	(destroy_block_symbol_cache): New function.
	(symbol_cache:~symbol_cache) Call destroy_block_symbol_cache.
	(resize_symbol_cache): Likewise.