api/buffer: Fix PVS/V547: use correct border for lnum

Should actually be silencing that for the sake of the case when `long` is 
actually not 64-bit. But it appears that Vim had already defined maximal line 
number. And even declared that exact value invalid, so no need in silencing.

src/nvim/pos.h

@@ -10,8 +10,10 @@ typedef int colnr_T;
 /// Format used to print values which have colnr_T type
 #define PRIdCOLNR "d"
 
-#define MAXLNUM 0x7fffffff     // maximum (invalid) line number
-#define MAXCOL  0x7fffffff     // maximum column number, 31 bits
+/// Maximal (invalid) line number
+enum { MAXLNUM = 0x7fffffff };
+/// Maximal column number, 31 bits
+enum { MAXCOL = 0x7fffffff };
 
 /*
  * position in file or buffer