Fix coverity defects: 147480, 147584

Fix coverity defects: 147480, 147584

CID 147480: Logically dead code (DEADCODE)

Remove non-null check and subsequent function call. Add ASSERT to future
proof the code.

usage label is only jumped to before `zhp` is initialized.

CID 147584: Out-of-bounds access (OVERRUN)

Subtract length of current string from buffer length for `size` argument
to `snprintf`.

Starting address for the write is the start of the buffer + the current
string length. We need to subtract this string length else risk a buffer
overflow.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tobin C. Harding <me@tobin.cc>
Closes #6745