MINOR: proxy: improve code when checking server name conflicts

MINOR: proxy: improve code when checking server name conflicts

During proxy_finalize(), a lookup is performed over the servers by name
tree to detect any collision. Only the first conflict for each server
instance is reported to avoid a combinatory explosion with too many
alerts shown.

Previously, this was written using a for loop without any iteration.
Replace this by a simple if statement as this is cleaner.

This should fix github issue #3276.