[PATCH][BZ #15698] Fix memory overrun in getifaddrs_internal.

[Ondřej Bílka]

Hi, a code at https://sourceware.org/bugzilla/show_bug.cgi?id=15698
contains a simple off-by-one error when preflen is divisible by 8.

Following code should fix this, as preflen is unsigned I added check for
zero len to be sure we do not cause underflow.

OK to commit?

	* sysdeps/unix/sysv/linux/ifaddrs.c (getifaddrs_internal): Fix
	memory overrun.

diff --git a/sysdeps/unix/sysv/linux/ifaddrs.c b/sysdeps/unix/sysv/linux/ifaddrs.c
index 89fda15..09676de 100644
--- a/sysdeps/unix/sysv/linux/ifaddrs.c
+++ b/sysdeps/unix/sysv/linux/ifaddrs.c
@@ -780,7 +780,7 @@ getifaddrs_internal (struct ifaddrs **ifap)
 		      else
 			preflen = ifam->ifa_prefixlen;
 
-		      for (i = 0; i < (preflen / 8); i++)
+		      for (i = 0; preflen && i < ((preflen - 1) / 8); i++)
 			*cp++ = 0xff;
 		      c = 0xff;
 		      c <<= (8 - (preflen % 8));