patch 9.0.1899: potential buffer overflow in PBYTE macro

patch 9.0.1899: potential buffer overflow in PBYTE macro

Problem:  potential buffer overflow in PBYTE macro
Solution: Check returned memline length

closes: #13083

the PBYTE macro is used to put byte c at a position lp of the returned
memline. However, in case of unexpected errors ml_get_buf() may return
either "???" or an empty line in which case it is quite likely that we
are causing a buffer overrun.

Therefore, switch the macro PBYTE (which is only used in ops.c anyhow)
to a function, that verifies that we will only try to access within the
given length of the buffer.

Also, since the macro is only used in ops.c, move the definition from
macros.h to ops.c

Signed-off-by: Christian Brabandt <cb@256bit.org>