proto: Factor out return in handle #5304

Job	Run time
test on freebsd	4m 10s
test on netbsd	5m 12s
audit	25s
lint	1m 4s
msrv	1m 9s
test (ubuntu-latest, stable)	1m 32s
test (ubuntu-latest, beta)	1m 41s
test (macos-latest, stable)	1m 11s
test-aws-lc-rs	2m 0s
test (windows-latest, stable)	4m 23s
test on illumos	6m 40s
test on solaris	6m 25s
test wasm32-unknown-unknown	52s
features (ubuntu-latest)	1h 21m 21s
features (macos-latest)	27m 27s
features (windows-latest)	53m 21s
test-android (x86_64-linux-android, x86_64, 26)	2m 52s
test-android (x86_64-linux-android, x86_64, 25)	2m 41s
test-android (i686-linux-android, x86, 26)	2m 39s
test-android (i686-linux-android, x86, 25)	3m 29s
	3h 30m 34s

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