ofproto: Fix potential NULL pointer dereference in ofproto_type_xx(). #6829

Job	Run time
dpdk gcc (ubuntu-24.04)	5s
linux rpm fedora	4m 3s
linux deb no dpdk	6m 41s
build oss-fuzz fuzzers	2m 52s
libreswan (ubuntu-24.04)	5s
osx clang --disable-ssl	2m 6s
clang-analyze-cache	10m 41s
linux gcc --disable-ssl	2m 29s
linux clang --disable-ssl	2m 44s
linux gcc c99	2m 43s
linux clang c99	2m 44s
linux gcc test	8m 29s
linux clang address,undefined test	16m 45s
linux gcc test --enable-shared	11m 21s
linux clang test --enable-shared	11m 14s
linux gcc check check-dpdk dpdk	19m 54s
linux clang check check-dpdk dpdk	19m 57s
linux gcc test -ljemalloc	9m 58s
linux clang test -ljemalloc	9m 2s
linux gcc --enable-afxdp	3m 59s
linux clang --enable-afxdp	2m 45s
linux gcc dpdk --enable-shared	3m 44s
linux clang dpdk --enable-shared	4m 44s
linux gcc dpdk-shared	2m 57s
linux clang dpdk-shared	3m 55s
linux gcc dpdk-shared --enable-shared	3m 44s
linux clang dpdk-shared --enable-shared	4m 13s
linux gcc m32 --disable-ssl	2m 47s
linux gcc check-ovsdb-cluster	6m 50s
linux gcc check-kernel -100	10m 17s
linux gcc check-kernel 100-	17m 19s
linux clang address,undefined check-kernel -100	12m 50s
linux clang address,undefined check-kernel 100-	19m 52s
linux gcc check-offloads -100	11m 0s
linux gcc check-offloads 100-	12m 38s
linux gcc dpdk check-system-userspace	12m 53s
linux clang address,undefined dpdk check-system-userspace	18m 22s
linux gcc dpdk check-system-tso	12m 40s
linux gcc dpdk check-afxdp	12m 38s
clang-analyze	10m 44s
	5h 34m 44s

Provide feedback