vm_eval.c (rb_adjust_argv_kw_splat): avoid memcpy with zero length

A method call is often with `argc = 1` and `argv = &v` where v is a
VALUE, and some functions shift the arguments by `argc-1` and `argv+1`
(for example, rb_sym_proc_call).  I'm unsure whether it is safe or not
to pass a pointer `argv+1` to memcpy with zero length, but Coverity Scan
complains it.  So this attempts to suppress the warning by explicit
check of the length.

vm_eval.c

@@ -244,7 +244,7 @@ rb_adjust_argv_kw_splat(int *argc, const VALUE **argv, int *kw_splat)
             int n = *argc;
             VALUE v;
             VALUE *ptr = rb_alloc_tmp_buffer2(&v, n+1, sizeof(VALUE));
-            memcpy(ptr, *argv, sizeof(VALUE)*n);
+            if (n) memcpy(ptr, *argv, sizeof(VALUE)*n);
             ptr[n] = rb_hash_new();
             *argc = ++n;
             *argv = ptr;