i2ctransfer: Zero out memory passed to ioctl()

Valgrind complains that uninitialized memory may be passed to
ioctl():

== Syscall param ioctl(I2C_RDWR) points to uninitialised byte(s)
==    at 0x499382B: ioctl (in /lib64/libc.so.6)
==    by 0x401957: main (i2ctransfer.c:343)
==  Address 0x1ffefff94c is on thread 1's stack
==  in frame #1, created by main (i2ctransfer.c:144)
==
== Syscall param ioctl(I2C_RDWR).msgs points to uninitialised byte(s)
==    at 0x499382B: ioctl (in /lib64/libc.so.6)
==    by 0x401957: main (i2ctransfer.c:343)
==  Address 0x1ffefff956 is on thread 1's stack
==  in frame #1, created by main (i2ctransfer.c:144)

Zero out the i2c_rdwr_ioctl_data struct as well as the msgs array to
guarantee that no uninitialized memory will ever be passed to the
kernel.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>