Also, consider the fact that even if you work on this company for six months and then fail/quit, employers won't think "this guy only worked at AmaGooFaceSoft for 6 months." They will think "this guy took initiative and spent 6 months building a cool product." You will positively impact your resume by building the business more than you will negatively impact it by leaving your current job before 1 year.
My advice is to ask yourself why you are scared. That seems like a signal that you do not believe in your product. If you are not 100% confident you can succeed, you shouldn't even start. Perhaps you should spend the next 6 months at your job refining the product idea and building up a bank balance (salary + options once they vest) to give yourself a 6+ month runway once you quit.
Also, don't be afraid of raising investment to cover your living expenses and the opportunity cost of spending the next 6 months at your job. You don't need to start with a million dollar round. You can raise enough to cover your living expenses ($100-250k) from angels via convertible debt. You could do this quite easily given your AmaGooFaceSoft background. Angel investors don't care how long you worked there, but they see it as a positive signal that AmaGooFaceSoft hired you in the first place. On the flipside, they would not see it as a positive signal if you are not 100% confident in your product.
Feel free to email me. I hate large corporations and am always happy to convince people to leave them.