svf: off-by-one error; do not access after the allocated memory

Keep the promise and ensure there're at least 3 bytes available after
the current position.

This eliminates the errors reported by Valgrind.

Change-Id: I1d0640e904c750eed808b2b4caf419b4d7619845
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1615


Tested-by: jenkins
Reviewed-by: Peter Stuge <peter@stuge.se>