mg - Check pointer before calling showbuffer()

[Mark Lumsden]

Source Joachim Nilsson:

    Found by Coverity Scan.  The popbuf() function iterated over a list to
    find a wp pointer, then sent it to showbuffer() which immediately went
    ahead and dereferenced it.  This patch simply adds a NULL pointer check
    before calling showbuffer(), if NULL then just return NULL to callee.

The missing NULL check is actually referenced in a comment a few lines
earlier in the code. ok?

-lum

Index: buffer.c
===================================================================
RCS file: /cvs/src/usr.bin/mg/buffer.c,v
retrieving revision 1.101
diff -u -p -u -p -r1.101 buffer.c
--- buffer.c    31 Aug 2016 12:22:28 -0000      1.101
+++ buffer.c    6 Sep 2016 17:04:22 -0000
@@ -713,12 +713,16 @@ popbuf(struct buffer *bp, int flags)
 
                while (wp != NULL && wp == curwp)
                        wp = wp->w_wndp;
-       } else
+       } else {
                for (wp = wheadp; wp != NULL; wp = wp->w_wndp)
                        if (wp->w_bufp == bp) {
                                wp->w_rflag |= WFFULL | WFFRAME;
                                return (wp);
                        }
+       }
+       if (!wp)
+               return (NULL);
+
        if (showbuffer(bp, wp, WFFULL) != TRUE)
                return (NULL);
        return (wp);