process.c: guard eargp against GC

`rb_str_append` may trigger GC, and in that case eargp might be GCed.
Probably for protecting it, `RB_GC_GUARD(execarg_obj)` can be seen in
other places.

Hoping to fix:
http://ci.rvm.jp/results/trunk_gcc5@silicon-docker/569818

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62570 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

process.c

@@ -4111,6 +4111,7 @@ rb_f_system(int argc, VALUE *argv)
         rb_str_cat_cstr(pst_message_status(str, status), ": ");
         rb_str_append(str, eargp->invoke.sh.shell_script);
         rb_exc_raise(rb_exc_new_str(rb_eRuntimeError, str));
+        RB_GC_GUARD(execarg_obj);
     }
     else {
         return Qfalse;