Solution (source code)

= Solution

For any $z\notin\{0,-1,-2,\ldots\}$, choose $N$ with $\operatorname{Re}(z+N)>0$ and define
$$
B(p,z)=\frac{(p+z)(p+z+1)\cdots(p+z+N-1)}
{z(z+1)\cdots(z+N-1)}B(p,z+N).
$$
The recurrence makes this independent of $N$ and agrees with the original integral where $\operatorname{Re}z>0$. It is the desired analytic continuation away from the listed points.