Silence gcc buffer overflow warning

[jamesjer]

GCC sees that an int will be written into service and knows that writing -2147483647 takes 12 bytes, so it warns that the sprintf call may overflow the 10 byte buffer. We know that port numbers don't cover the entire range of values of the int type, so the code is not really wrong. This patch is purely to make gcc be quiet.

Note that on modern CPU architectures, either 12 or 16 bytes will be allocated anyway, for alignment reasons, so this change doesn't increase stack usage.