Fix heap-buffer-overflow in `pythonbuf` with undersized buffers (#6019)

* Fix heap-buffer-overflow in pythonbuf with undersized buffers (gh-5886)

The _sync() UTF-8 remainder logic can leave pptr() past the end of
the allocated buffer when buf_size < 4: after moving up to 3 bytes
of an incomplete UTF-8 sequence to the front, pbump(remainder) pushes
pptr() beyond epptr() and the buffer boundary.  The next overflow()
then writes out of bounds.

Fix by clamping the buffer size to a minimum of 4 in the constructor,
ensuring the maximum UTF-8 remainder (3 bytes) plus the overflow slot
(1 byte) always fits within the allocated buffer.

Made-with: Cursor

* Avoid C++14 ODR-use linker error for minimum_buffer_size

std::max takes arguments by const&, which ODR-uses the static constexpr
member and requires an out-of-line definition in C++14. Replace with a
ternary expression that uses the value without taking its address.

Made-with: Cursor