Avoid holding onto the buffer when parsing unknown length-delimited fields #1136

Job	Run time
format_analyze; linux; Dart 2.19.0; PKG: protobuf; `dart analyze lib`, `dart analyze test`	31s
format_analyze; linux; Dart dev; PKG: benchmarks; `dart format --output=none --set-exit-if-change...	32s
format_analyze; linux; Dart dev; PKG: api_benchmark; `dart format --output=none --set-exit-if-cha...	45s
mono_repo self validate	19s
format_analyze; linux; Dart dev; PKG: protobuf; `dart format --output=none --set-exit-if-changed ...	24s
format_analyze; linux; Dart dev; PKG: protoc_plugin; `dart format --output=none --set-exit-if-cha...	30s
run_tests; linux; Dart dev; PKG: protobuf; `dart test`	47s
run_tests; linux; Dart 2.19.0; PKG: protoc_plugin; `./../tool/setup.sh`, `make protos`, `dart test`	52s
run_tests; linux; Dart 2.19.0; PKG: protobuf; `dart test`	33s
run_tests; linux; Dart dev; PKG: protoc_plugin; `./../tool/setup.sh`, `make protos`, `dart test`	1m 1s
run_tests; osx; Dart 2.19.0; PKG: protobuf; `dart test`	1m 27s
run_tests; osx; Dart dev; PKG: protobuf; `dart test`	1m 16s
run_tests; windows; Dart 2.19.0; PKG: protobuf; `dart test`	42s
run_tests; windows; Dart dev; PKG: protobuf; `dart test`	52s
run_legacy_tests; linux; Dart 2.19.0; PKG: protoc_plugin; `./../tool/setup.sh`, `make protos`, `d...	38s
	11m 9s

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