Fix a case of undefined behavior due to overlapping buf objects in snprintf (buf, size, fmt, buf, etc). This only works by chance with our libc, but fails (with a truncated string) on e.g. glibc.
Fix a case of undefined behavior due to overlapping buf objects in
snprintf (buf, size, fmt, buf, etc). This only works by chance with our
libc, but fails (with a truncated string) on e.g. glibc.

Okayed by:	sobomax
MFC after:	1 week

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