Avoid assertion failure on after(k) or older(k) where k < 1 or k >= 0x80000000UL

[practicalswift]

Before:

$ ./miniscript <<< "after(0)"
miniscript: bitcoin/script/miniscript.h:366: miniscript::Type miniscript::Node<Key>::CalcType() const [with Key = std::__cxx11::basic_string<char>]: Assertion `k >= 1 && k < 0x80000000UL' failed.
Aborted
$ ./miniscript <<< "older(0)"
miniscript: bitcoin/script/miniscript.h:366: miniscript::Type miniscript::Node<Key>::CalcType() const [with Key = std::__cxx11::basic_string<char>]: Assertion `k >= 1 && k < 0x80000000UL' failed.
Aborted

After:

$ ./miniscript <<< "after(0)"
Failed to parse as policy or miniscript 'after(0)'
$ ./miniscript <<< "older(0)"
Failed to parse as policy or miniscript 'older(0)'

[sipa]

ACK!