fad231ad41 Fix merging of global unknown data in PSBTs (Andrew Chow)
41df035ee1 Check that PSBT keys are the correct length (Andrew Chow)
Pull request description:
This PR fixes a few bugs that were found and adds tests checking for these errors.
Specifically:
- Single byte keys are checked to actually be one byte.
- Unknown global data must be merged when combining two PSBTs.
Tree-SHA512: c0e7b4bc607d510cc005aaa7c0813ee58c5467ab7ce4adce485522dfeee92b1af3d29fe89df778b0ea812bb3827e085b30e04d4f4ebcefd8364d809573991332