crypto_get_ptrs() should always write to *out_data_2

Callers will check if it has been set to NULL before trying to access
it, but never initialize it themselves. Whenever "one block spans two
iovecs", crypto_get_ptrs() will return, without ever setting
*out_data_2 = NULL. The caller will then do a NULL check against the
uninitailized pointer and if it is not zero, pass it to memcpy().

The only reason this has not caused horrible runtime issues is because
memcpy() should be told to copy zero bytes when this happens. That
said, this is technically undefined behavior, so we should correct it so
that future changes to the code cannot trigger it.

Clang's static analyzer found this with the help of CodeChecker's CTU
analysis.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14043