BIP-119: Fix missing self. prefix on stack reference in pseudocode

[schjonhaug]

Summary

In the execute_bip_119 pseudocode, the hash comparison references stack[-1] instead of self.stack[-1], inconsistent with all other stack references in the function:

# Correct:
if len(self.stack) < 1:
if len(self.stack[-1]) == 32:

# Incorrect — missing self.:
if stack[-1] != self.context.tx.get_default_check_template_hash(...)

In Python, stack[-1] would reference an undefined local variable, while self.stack[-1] correctly accesses the interpreter's stack. The C++ reference implementation uses stack.back() consistently and is not affected.