This:
- Allows `.*btc` amounts (without post-decimal)
- Avoids creating decimals when amount is 0 btc
- Corrects our handling of the suffixes (memeqstr would
sometimes return false because of null-termination)
Changelog-Fixed: We are now able to parse any amount string (XXXmsat, XX.XXXbtc, ..) we create.
Signed-off-by: Antoine Poinsot <darosior@protonmail.com>