Fix memory leak when RUBYOPT is invalid

When RUBYOPT is invalid, it raises an error which causes moreswitches
to leak memory. It can be seen when building with LSAN enabled:

    $ RUBY_FREE_AT_EXIT=1 RUBYOPT=f ruby
    ruby: invalid option -f  (-h will show valid options) (RuntimeError)

    Direct leak of 16 byte(s) in 1 object(s) allocated from:
        #0 0x618cef8efa23 in malloc (miniruby+0x64a23)
        #1 0x618cefa0e8d8 in rb_gc_impl_malloc gc/default/default.c:8182:5
        #2 0x618cef9f7f01 in ruby_xmalloc2_body gc.c:5182:12
        #3 0x618cef9f7eac in ruby_xmalloc2 gc.c:5176:34
        #4 0x618cefb547b2 in moreswitches ruby.c:919:18
        #5 0x618cefb526fe in process_options ruby.c:2350:9
        #6 0x618cefb524ac in ruby_process_options ruby.c:3202:12
        #7 0x618cef9dc11f in ruby_options eval.c:119:16
        #8 0x618cef8f2fb5 in rb_main main.c:42:26
        #9 0x618cef8f2f59 in main main.c:62:12

ruby.c

@@ -916,7 +916,9 @@ moreswitches(const char *s, ruby_cmdline_options_t *opt, int envopt)
     argc = RSTRING_LEN(argary) / sizeof(ap);
     ap = 0;
     rb_str_cat(argary, (char *)&ap, sizeof(ap));
-    argv = ptr = ALLOC_N(char *, argc);
+
+    VALUE ptr_obj;
+    argv = ptr = RB_ALLOCV_N(char *, ptr_obj, argc);
     MEMMOVE(argv, RSTRING_PTR(argary), char *, argc);
 
     while ((i = proc_options(argc, argv, opt, envopt)) > 1 && envopt && (argc -= i) > 0) {
@@ -948,7 +950,8 @@ moreswitches(const char *s, ruby_cmdline_options_t *opt, int envopt)
         opt->crash_report = crash_report;
     }
 
-    ruby_xfree(ptr);
+    RB_ALLOCV_END(ptr_obj);
+
     /* get rid of GC */
     rb_str_resize(argary, 0);
     rb_str_resize(argstr, 0);