If the returned buffer string is of length 0, then a realloc can happen
with length 0. However, the behaviour is implementation-defined.
From 7.20.3.1 of C11 spec:

> If the size of the space requested is zero, the behavior is
> implementation-defined: either a null pointer is returned,
> or the behavior is as if the size were some nonzero value,
> except that the returned pointer shall not be used to access an object

This is problematic for the test case on my system as it returns NULL,
causing a memleak and later using it in memcpy causing UB.
The bucket code is not prepared to handle a NULL pointer.
To solve this, we use MAX to clamp the size to 1 at the least.

Closes GH-17656.