theres a bug is in sclose() where it doesnt check if wp is beyond
the buffer. also wp was not updated after realloc().
bug was reported by porlock on 9fans:
Plan 9's implementation of the standard C functions snprintf and
vsnprintf have a buffer overrun bug.
If the buffer length equals the output length (without the terminating
null), then one too many characters is written to the buffer.
For example,
snprintf(buf, 4, "ABCD");
will write 5 characters to buf.
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