Why the share value of the company does not goes in to negative value?

Good question.

In case of Equity shares the liability of shares is extend to the unpaid amount on it maximum.

So when you purchase shares you will not get additional consideration also along with it--as you have no obligation to meet by way of any liability,  as a consequence of holding such shares.

Buying of shares is just like buying of commodities it will have some value but not a negative value.