Null pointer dereference issue in function ub_ctx_set_fwd of file libunbound/libunbound.c

[xiaoxiaoafeifei]

Describe the bug
Hi, I found a null pointer dereference issue in function ub_ctx_set_fwd of file libunbound/libunbound.c

To reproduce
Steps to reproduce the behavior:

1. export CFLAGS="-g -O0 -fsanitize=address,undefined"
   export CC=afl-clang-fast
   ./configure
   make && make install
2. /usr/local/sbin/unbound-host -C poc_file -r test
   poc_file:
   poc_file.zip

Evidence
==1980801==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x55d39d62f8c0 bp 0x7ffeefa39db0 sp 0x7ffeefa39550 T0)
==1980801==The signal is caused by a READ memory access.
==1980801==Hint: address points to the zero page.
#0 0x55d39d62f8c0 in __interceptor_strcmp (/usr/local/sbin/unbound-host+0x718c0) (BuildId: dfa431122871d2dc572574bc91716352a41072a7)
#1 0x7f1406a5d379 in ub_ctx_set_fwd /root/fuzz/fuzz_unbound/unbound_1/libunbound/libunbound.c:1004:3
#2 0x7f1406a65010 in ub_ctx_resolvconf /root/fuzz/fuzz_unbound/unbound_1/libunbound/libunbound.c:1205:12
#3 0x55d39d6ed5a6 in main /root/fuzz/fuzz_unbound/unbound/smallapp/unbound-host.c:465:17
#4 0x7f1405ea1d8f in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
#5 0x7f1405ea1e3f in __libc_start_main csu/../csu/libc-start.c:392:3
#6 0x55d39d6186e4 in _start (/usr/local/sbin/unbound-host+0x5a6e4) (BuildId: dfa431122871d2dc572574bc91716352a41072a7)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV (/usr/local/sbin/unbound-host+0x718c0) (BuildId: dfa431122871d2dc572574bc91716352a41072a7) in __interceptor_strcmp
==1980801==ABORTING

System:
Unbound version: 1.20.0
OS: Ubuntu 22.04.3 LTS