pset: fix pegin/issuance flag handling in extract_tx #647

Job	Run time
Check WASM	32s
Docs - stable toolchain (recent)	41s
Test - stable toolchain	2m 41s
Prepare	5s
Docs - nightly toolchain (recent)	42s
Lint - nightly toolchain (recent)	44s
Test - MSRV	8m 11s
Test - nightly toolchain	2m 39s
	16m 15s

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