bfd: avoid out-of-bound access

Write a nullbyte only if there is enought space for it.

Cc: Stephen Röttger <stephen.roettger@gmail.com>
Reported-by: Stephen Röttger <stephen.roettger@gmail.com>
Signed-off-by: Andrei Vagin <avagin@openvz.org>

criu/bfd.c

@@ -196,6 +196,11 @@ again:
 		if (!b->sz)
 			return NULL;
 
+		if (b->sz == BUFSIZE) {
+			pr_err("The bfd buffer is too small\n");
+			ERR_PTR(-EIO);
+			return NULL;
+		}
 		/*
 		 * Last bytes may lack the \n at the
 		 * end, need to report this as full