glibc: incorrect use of extend_alloca() in formatted printing can lead to FORTIFY_SOURCE format string protection bypass

The vfprintf function in stdio-common/vfprintf.c in libc in GNU C Library aka glibc 2.14 and other versions does not properly calculate a buffer length, which allows context-dependent attackers to bypass the FORTIFYSOURCE format-string protection mechanism and cause a denial of service segmentati...