[SV 65324] disable_builtins: Don't dereference NULL suffix_file

Make crashes when -r and MAKEFLAGS= are specified on the command line.

On startup make begins to process command line arguments.
During processing of "MAKEFLAGS=" make calls reset_makeflags, which in
turn calls disable_builtins, which dereferences null suffix_file.
Here is the backtrace.

0 disable_builtins main.c:3482
1 reset_makeflags main.c:3104
2 set_special_var variable.c:1325
3 do_variable_definition variable.c:1693
4 try_variable_definition variable.c:1889
5 handle_non_switch_argument main.c:3021
6 decode_switches main.c:3150
7 main main.c:1621

* src/main.c (disable_builtins): Avoid dereferencing null suffix_file.
* tests/scripts/features/suffixrules: Add a test.