codegen: Fix segfault of CodegenLLVM::generateProbe() #3

Job	Run time
build_test (LLVM 12 Release, Release, .#bpftrace-llvm12, biosnoop.bt)	15m 11s
clang-format	4m 12s
build_test (LLVM 13 Release, Release, .#bpftrace-llvm13, biosnoop.bt)	14m 33s
build_test (LLVM 14 Release, Release, .#bpftrace-llvm14, biosnoop.bt)	14m 40s
build_test (LLVM 15 Release, Release, .#bpftrace-llvm15, biosnoop.bt)	14m 48s
build_test (LLVM 16 Release, Release, .#bpftrace-llvm16, biosnoop.bt)	14m 39s
build_test (LLVM 17 Release, Release, .#bpftrace-llvm17, biosnoop.bt)	14m 31s
build_test (LLVM 18 Release, Release, .#bpftrace-llvm18, biosnoop.bt)	14m 24s
build_test (LLVM 18 Debug, Debug, .#bpftrace-llvm18, biosnoop.bt)	15m 52s
build_test (LLVM 18 Clang Debug, Debug, .#bpftrace-llvm18, clang, clang++, biosnoop.bt)	15m 12s
build_test (Memleak test (LLVM 18 Debug), Debug, .#bpftrace-llvm18, 1, 0)	9m 50s
build_test (Memleak test (LLVM 18 Release), Release, .#bpftrace-llvm18, 1, 0)	8m 28s
irc	0s
	2h 36m 20s

Provide feedback