fix: add NULL check for GvIOp in XML_Do_External_Parse

[toddr-bot]

What

Prevent segfault when ExternEnt handler returns an unopened filehandle (glob with no IO slot).

Why

GvIOp() returns NULL when a glob has never been opened. Both call sites in XML_Do_External_Parse (lexical glob ref at L2357 and bare glob at L2361) passed the result directly to newRV_inc(NULL), which dereferences the NULL pointer — instant segfault. While most handlers return properly opened filehandles, a buggy or incomplete handler could trigger this crash with no useful diagnostic.

How

Extract the GvIOp() result into a local IO *io variable and check for NULL before use. On NULL, croak() with a descriptive message instead of dereferencing.

Testing

• Added 2 tests to t/extern_ent_lexical_glob.t: one for bare unopened glob, one for scalar ref (different code path but same area)
• Full test suite passes

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.45%. Comparing base (45216a7) to head (04b2db8).

Files with missing lines	Patch %	Lines
Expat/Expat.xs	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #259      +/-   ##
==========================================
+ Coverage   75.73%   76.45%   +0.72%     
==========================================
  Files           1        1              
  Lines        1092     1096       +4     
  Branches      342      344       +2     
==========================================
+ Hits          827      838      +11     
+ Misses         59       52       -7     
  Partials      206      206              

Flag	Coverage Δ
perl	76.45% <66.66%> (+0.72%)	⬆️
xs	76.45% <66.66%> (+0.72%)	⬆️

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