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>

src/ops.c

@@ -17,6 +17,9 @@
 static void shift_block(oparg_T *oap, int amount);
 static void	mb_adjust_opend(oparg_T *oap);
 static int	do_addsub(int op_type, pos_T *pos, int length, linenr_T Prenum1);
+static void	pbyte(pos_T lp, int c);
+#define PBYTE(lp, c) pbyte(lp, c)
+
 
 // Flags for third item in "opchars".
 #define OPF_LINES  1	// operator always works on lines
@@ -4349,3 +4352,18 @@ do_pending_operator(cmdarg_T *cap, int old_col, int gui_yank)
     restore_lbr(lbr_saved);
 #endif
 }
+
+// put byte 'c' at position 'lp', but
+// verify, that the position to place
+// is actually safe
+    static void
+pbyte(pos_T lp, int c)
+{
+    char_u *p = ml_get_buf(curbuf, lp.lnum, TRUE);
+    int	len = curbuf->b_ml.ml_line_len;
+
+    // safety check
+    if (lp.col >= len)
+	lp.col = (len > 1 ? len - 2 : 0);
+    *(p + lp.col) = c;
+}