Allow op-cache winner in non-critical race condition

Hi John, I'm experiencing a situation where two or more threads attempt to add objects with the same hash value to the operation cache.

> VIPS:ERROR:cache.c:606:vips_cache_insert: assertion failed: (!g_hash_table_lookup( vips_cache_table, operation ))

I think it's due to the (deadlock-prevention?) gap between an unlock/lock pair in `vips_cache_operation_buildp` - https://github.com/jcupitt/libvips/blob/master/libvips/iofuncs/cache.c#L762-L779

As operation objects with the same hash value are effectively equal, this PR changes the situation from a critical to non-critical race condition, allowing the first thread to add a given operation to "win".

(`g_new` is moved below `g_hash_table_lookup` to prevent a memory leak.)