psbt: avoid assigning non-boolean values to bool (ubsan)

ubsan doesn't like assigning arbitrary uint8_t values to bool. It's easy
to avoid doing, so do it. (We do this specifically in PSET since that's
Elements-specific code, but the same issue is present in Bitcoin in the
Unserialize impl for bool in serialize.h. Upstream this is only used in
the wallet database, where it may be that non 0/1 values are impossible
(absent a corrupt wallet).

Author: apoelstra

src/psbt.h

@@ -1091,9 +1091,9 @@ struct PSBTInput
                                 } else if (subkey_len != 1) {
                                     throw std::ios_base::failure("Input issuance needs blinded flag is more than one byte type");
                                 }
-                                bool b;
+                                uint8_t b;
                                 UnserializeFromVector(s, b);
-                                m_blinded_issuance = b;
+                                m_blinded_issuance = !!b;
                                 break;
                             }
                             default: