Fix panic in user defined aggregation functions planning #141

Job	Run time
Set push variable	1s
Build and push vttestserver (mysql80)	2m 10s
Build and push lite (latest)	2m 22s
Build and push lite (percona80)	2m 20s
Build and push (bullseye, vtadmin)	1m 18s
Build and push (bullseye, vtorc)	39s
Build and push (bullseye, vtgate)	35s
Build and push (bullseye, vttablet)	37s
Build and push (bullseye, mysqlctld)	33s
Build and push (bullseye, mysqlctl)	51s
Build and push (bullseye, vtctl)	28s
Build and push (bullseye, vtctlclient)	37s
Build and push (bullseye, vtctld)	1m 16s
Build and push (bullseye, vtctldclient)	45s
Build and push (bullseye, logrotate)	30s
Build and push (bullseye, logtail)	26s
Build and push (bullseye, vtbackup)	33s
Build and push (bullseye, vtexplain)	39s
Build and push (bookworm, vtadmin)	1m 6s
Build and push (bookworm, vtorc)	40s
Build and push (bookworm, vtgate)	33s
Build and push (bookworm, vttablet)	46s
Build and push (bookworm, mysqlctld)	49s
Build and push (bookworm, mysqlctl)	42s
Build and push (bookworm, vtctl)	41s
Build and push (bookworm, vtctlclient)	35s
Build and push (bookworm, vtctld)	1m 22s
Build and push (bookworm, vtctldclient)	41s
Build and push (bookworm, logrotate)	30s
Build and push (bookworm, logtail)	22s
Build and push (bookworm, vtbackup)	41s
Build and push (bookworm, vtexplain)	35s
	26m 43s

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