Fix segmentation fault in modtile.c:delay_allowed (#395) #523

Job	Run time
freebsd13 (CMake) (LLVM)	7m 34s
debian:12 (Latest Mapnik) (CMake) (GNU)	3m 14s
ubuntu:22.04 (Latest Mapnik) (CMake) (GNU)	3m 4s
freebsd14 (CMake) (LLVM)	6m 25s
debian:11 (CMake) (GNU)	3m 4s
macos-12 (CMake) (LLVM)	10m 37s
debian:12 (CMake) (GNU)	3m 4s
macos-13 (Autotools) (LLVM)	10m 15s
debian:testing (CMake) (GNU)	3m 0s
macos-13 (CMake) (LLVM)	11m 51s
fedora:38 (CMake) (GNU)	4m 42s
fedora:39 (CMake) (GNU)	4m 49s
fedora:rawhide (CMake) (GNU)	3m 0s
opensuse/leap:15 (CMake) (GNU)	6m 30s
quay.io/centos/centos:stream8 (CMake) (GNU)	4m 38s
quay.io/centos/centos:stream9 (CMake) (GNU)	5m 13s
ubuntu:20.04 (CMake) (GNU)	3m 19s
ubuntu:rolling (CMake) (GNU)	2m 41s
ubuntu:22.04 (Autotools) (GNU)	1m 53s
ubuntu:22.04 (Autotools) (LLVM)	2m 11s
ubuntu:22.04 (CMake) (GNU)	3m 4s
ubuntu:22.04 (CMake) (LLVM)	2m 41s
	1h 46m 49s

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