BIP62: Make OP_0 a validly encoded signature

Previously BIP62 did not provide a compact way to delibrately encode an invalid signature. For example in BIP19 if m != n with this change you can provide compact OP_0's in the scriptSig rather than lengthy DER-encoded signatures. Note that we may want to further expand on this change in the future by saying that only OP_0 is a "valid" invalid signature; BIP19 even with this change is inherently malleable as the invalid signatures can be any validly encoded DER signature. CC: @sipa

Specification