coverity/13685: silence NUL-termnation fp

It was a false positive, but it can't hurt to "fix" it.

Original warning:
CID 13685 (#1 of 1): Buffer not null terminated (BUFFER_SIZE)
6. buffer_size: Calling strncpy with a source string whose length (4 chars)
is greater than or equal to the size argument (4) will fail to
null-terminate b0p->b0_version.

src/nvim/memline.c

@@ -327,8 +327,7 @@ int ml_open(buf_T *buf)
   b0p->b0_magic_int = (int)B0_MAGIC_INT;
   b0p->b0_magic_short = (short)B0_MAGIC_SHORT;
   b0p->b0_magic_char = B0_MAGIC_CHAR;
-  STRNCPY(b0p->b0_version, "VIM ", 4);
-  STRNCPY(b0p->b0_version + 4, Version, 6);
+  xstrlcpy(xstpcpy((char *) b0p->b0_version, "VIM"), Version, 6);
   long_to_char((long)mfp->mf_page_size, b0p->b0_page_size);
 
   if (!buf->b_spell) {