ber_ptrlen() should return ber_len_t

[masarati@aero.polimi.it]

In fact, it makes no sense that ber_ptr < ber_buf; if it happens, the
BerElement is corrupted, and an assertion failure may be more appropriate.
 Right now, no test is done, and the (trivial) function could be replaced
with a macro...  Since this is part of the private API, its change should
be harmless.  Also, it should be renamed ber_int_ptrlen().

p.