[Clang][Attr] fix a btf_type_attr CGDebugInfo codegen bug
Nathan Chancellor reported a crash due to commit
3466e00716e1 (Reland "[Attr] support btf_type_tag attribute").
The following test can reproduce the crash:
$ cat efi.i
typedef unsigned long efi_query_variable_info_t(int);
typedef struct {
struct {
efi_query_variable_info_t __attribute__((regparm(0))) * query_variable_info;
};
} efi_runtime_services_t;
efi_runtime_services_t efi_0;
$ clang -m32 -O2 -g -c -o /dev/null efi.i
The reason is that FunctionTypeLoc.getParam(Idx) may return a
nullptr which should be checked before dereferencing the
result pointer. This patch fixed this issue.