Fix UnboundLocalError in newick read_props when a required value is missing (#799)

[youdie006]

Fixes #799.

Problem

import ete4
ete4.Tree("(A);", parser=3)

crashes with:

UnboundLocalError: local variable 'p1_str' referenced before assignment

in ete4.parser.newick.read_props, on ete4 4.4.0 and current ete4 HEAD.

Root cause

In read_props (ete4/parser/newick.pyx), the second try block only binds p1_str inside its if branch (when a :-separated value is present). When a strict parser requires that value but it is absent, the elif check_req and p1_req branch raises AssertionError('missing required value') while p1_str is still unbound. The except handler then references p1_str (raise NewickError('parsing %r: %s' % (p1_str, e))), so instead of the intended NewickError the caller gets an UnboundLocalError. This fires for every strict parser with a required second field (parsers 2, 3, 5, 6, 7) when a required dist/support value is missing.

Fix

Initialize p1_str = '' before the try, mirroring how p0_str is always defined earlier in the same function. The error message now reads sensibly: parsing '': missing required value.

This is deliberately narrow: it does not make the invalid input parse. parser=3 still correctly rejects (A); (as @dengzq1234 notes in the issue) -- the change only ensures the parser raises the intended, catchable NewickError described in its documented error contract instead of crashing with an UnboundLocalError, which is exactly the reporter's open complaint ("that doesn't return an error, it just blows up").

Test

Added test_newick_missing_required_value to tests/test_tree.py, mirroring the existing newick error-test style, asserting Tree('(A);', parser=3) raises NewickError. Verified red-green against the rebuilt Cython extension: before the fix the test fails with UnboundLocalError; after it passes. Full tests/test_tree.py and adjacent newick-parsing modules pass with no regressions.

Thanks to @terrycojones for the clear report.

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This change was prepared with AI assistance and reviewed by me before submission.