sdssplitargs(): on error set *argc to 0.

This makes programs not checking the return value for NULL much safer
since with this change:

1) It is still possible to iterate the zero-length result without
crashes.
2) sdssplitargs_free will work against NULL and 0 count.

src/sds.c

@@ -593,6 +593,7 @@ err:
         sdsfree(vector[*argc]);
     zfree(vector);
     if (current) sdsfree(current);
+    *argc = 0;
     return NULL;
 }