Description:
------------
The fix for bug #72093 introduced split_bc_num(), but this
function appears to be pointless, because it returns its input if
the refcount is greater than or equal 1[1] (what' most likely
always true).

Even if the split would be done if the refcount is greater than 1,
it appears that the shallow copy[2] would not be sufficient
anyway, since bc_struct holds pointers to strings[3] which would
still be shared.

I have not been able to come up with a test case showing the
assumed misbehavior. Actually, all tests still pass (even with
valgrind enabled), if the function's body is changed to always
return num.

[1] <https://github.com/php/php-src/blob/php-7.2.0RC1/ext/bcmath/bcmath.c#L214-L216>
[2] <https://github.com/php/php-src/blob/php-7.2.0RC1/ext/bcmath/bcmath.c#L218>
[3] <https://github.com/php/php-src/blob/php-7.2.0RC1/ext/bcmath/libbcmath/src/bcmath.h#L46-L52>