add validation for when the command buffer before the response file gets truncated

While playing with a new toolchain I noticed that my command line got truncated. Finally I found out that it was the command before the respone file suffix that was too big for the current command_buf length.

I added a validation for it along what should be a clear message of what's happening and what to do about it and I stop the execution instead of letting it continue with a truncated command that won't probably work as expected (if at all).