Avoid -fsanitize=nullability in external/zlib

...as seen during CppunitTest_sw_layoutwriter:

> testUserFieldTypeLanguage::TestBody finished in: 1271ms
[...]
> workdir/UnpackedTarball/zlib/trees.c:873:42: runtime error: null pointer passed as argument 2, which is declared to never be null
> /usr/include/string.h:43:28: note: nonnull attribute specified here
>  #0 in _tr_stored_block at workdir/UnpackedTarball/zlib/trees.c:873:5
>  #1 in deflate at workdir/UnpackedTarball/zlib/deflate.c:1025:17
>  #2 in ZipUtils::ThreadedDeflater::Task::doWork() at package/source/zipapi/ThreadedDeflater.cxx:163:15
[...]

Change-Id: I7d4b68ab2c4fb3080fc48211236cb33ca4bf7126
Reviewed-on: https://gerrit.libreoffice.org/73143
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

external/zlib/UnpackedTarball_zlib.mk

@@ -16,4 +16,10 @@ $(eval $(call gb_UnpackedTarball_set_post_action,zlib,\
 	cp $(addsuffix .c,adler32 compress crc32 deflate inffast inflate inftrees trees zutil) x64 \
 ))
 
+$(eval $(call gb_UnpackedTarball_set_patchlevel,zlib,0))
+
+$(eval $(call gb_UnpackedTarball_add_patches,zlib, \
+    external/zlib/ubsan.patch \
+))
+
 # vim: set noet sw=4 ts=4:

external/zlib/ubsan.patch

+--- trees.c
++++ trees.c
+@@ -870,7 +870,7 @@
+     bi_windup(s);        /* align on byte boundary */
+     put_short(s, (ush)stored_len);
+     put_short(s, (ush)~stored_len);
+-    zmemcpy(s->pending_buf + s->pending, (Bytef *)buf, stored_len);
++    if (stored_len != 0) zmemcpy(s->pending_buf + s->pending, (Bytef *)buf, stored_len);
+     s->pending += stored_len;
+ #ifdef ZLIB_DEBUG
+     s->compressed_len = (s->compressed_len + 3 + 7) & (ulg)~7L;