pb: Init fsize to make compiler happy

protobuf.c: In function ‘pb_show_msg’:
protobuf.c:384:68: error: ‘fsize’ may be used uninitialized in this function [-Werror=uninitialized]
protobuf.c:247:9: note: ‘fsize’ was declared here

The warning about 'may be uninitialized' is false -- the BUG() is a never-returning function.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>