GH-16655 test failure in 8.3.26

Description

Building 8.3.26 from source and running its test suite, I get:

=====================================================================
TIME END 2025-09-30 00:53:03

=====================================================================
TEST RESULT SUMMARY
---------------------------------------------------------------------
Exts skipped    :    18 (zend_test, sysvshm, sysvmsg, soap, snmp, skeleton, shmop, pdo_odbc, pdo_oci, pdo_firebird, pdo_dblib, odbc, oci8, imap, ffi, enchant, dl_test, com_dotnet)
Exts tested     :    55
---------------------------------------------------------------------

Number of tests : 19484             15980
Tests skipped   :  3504 ( 18.0%) --------
Tests warned    :     1 (  0.0%) (  0.0%)
Tests failed    :     1 (  0.0%) (  0.0%)
Expected fail   :    12 (  0.1%) (  0.1%)
Tests passed    : 15966 ( 81.9%) ( 99.9%)
---------------------------------------------------------------------
Time taken      :    67 seconds
=====================================================================

=====================================================================
FAILED TEST SUMMARY
---------------------------------------------------------------------
GH-16559 (UBSan abort in ext/gd/libgd/gd_interpolation.c:1007) [ext/gd/tests/gh16559.phpt]
=====================================================================

=====================================================================
WARNED TEST SUMMARY
---------------------------------------------------------------------
FPM: Buffered worker output decorated log with multiple continuous messages (stdout/stderr mixed) [sapi/fpm/tests/log-bwd-multiple-msgs-stdout-stderr.phpt] (warn: XFAIL section but test passes)
=====================================================================

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make: *** [Makefile:249: test] Error 1

Test suite failed, dumping logs.
error: in phase 'check': uncaught exception:
%exception #<&invoke-error program: "make" arguments: ("test" "-j" "32" "--max-load=32") exit-status: 2 term-signal: #f stop-signal: #f> 
phase `check' failed after 68.1 seconds
command "make" "test" "-j" "32" "--max-load=32" failed with status 2

The inputs used to build the package:

aspell@0.60.8 bison@3.8.2 bzip2@1.0.8 curl@8.6.0 cyrus-sasl@2.1.28 fontconfig-minimal@2.14.0
+ freetype@2.13.3 gd@2.3.3 gdbm@1.25 gettext-minimal@0.23.1 gmp@6.3.0 gnutls@3.8.3 icu4c@73.1 libgcrypt@1.11.0
+ libjpeg-turbo@2.1.4 libpng@1.6.39 libsodium@1.0.18 libx11@1.8.12 libxml2@2.14.6 libxslt@1.1.43 libzip@1.9.2
+ oniguruma@6.9.10 openldap@2.6.9 openssl@3.0.8 pcre2@10.42 pkg-config@0.29.2 postgresql@14.13 procps@4.0.3
+ readline@8.2.13 sqlite@3.39.3 tidy-html@5.8.0 zlib@1.3.1

Expected no test failure.

PHP Version

8.3.26, built from source

Operating System

GNU Guix

Start by reproducing the reported failure with make test for PHP 8.3.26 and inspect ext/gd/tests/gh16559.phpt. Trace the UBSan abort reported at ext/gd/libgd/gd_interpolation.c:1007; done means the test suite no longer reports GH-16559 as failed.