Check for NULL before dereferencing a pointer

l is part of the exit condition of the while loop. If we exit the loop
because l is NULL, we cannot dereference it.

file.c

@@ -8139,6 +8139,7 @@ getNextPage(Buffer *buf, int plen)
 		l = l->next;
 	    } while (l && l->bpos);
 	    buf->firstLine = l;
+	    if (l)
 		buf->firstLine->prev = NULL;
 	}
     }