1/ Prevent an out-of-bounds write if sz is 0.
2/ Limit sz to INT_MAX, because the return value is an int, not an
unsigned int (and because if INT_MAX is equal to SSIZE_MAX, man 2 read
says "If count is greater than SSIZE_MAX, the result is unspecified.")
3/ Always null-terminate dst (unless sz is 0), because a return value of
0 because of an open() error (for example) is indistinguishable from a
return value of 0 because of an empty file.
4/ Use an unsigned int for i (just like n), not an int.
5/ Check for snprintf() truncation.
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