The counters for syncd (switch chip) were attempted to be added to
gbsyncd (gearbox phys), and vice versa.
This issue is introduced by
sonic-net#1362
When setting the redis attribute
SAI_REDIS_SWITCH_ATTR_FLEX_COUNTER_GROUP and
SAI_REDIS_SWITCH_ATTR_FLEX_COUNTER, the operation is applied to every
contexts (both syncd and gbsyncd). However, the counters to initialize
could only exist in one context.

The fix is to check that the target switch id exists in the context; if
not, skip the operation.

Normally, setting SAI_REDIS_SWITCH_ATTR_FLEX_COUNTER_GROUP and
SAI_REDIS_SWITCH_ATTR_FLEX_COUNTER should provide a valid switch id
and the setting is only applied to the context with the switch id.
Fall back to loop all contexts if SAI_NULL_OBJECT_ID is provided.