Remove redundant PSK length check.

If psk_len were 0, it would already have been an error earlier. The PSK cipher
suites don't lose the other_secret || psk construction if the PSK happens to be
empty.

Change-Id: I1917236720d0862658562bc8f014cb827ee9aed5
Reviewed-on: https://boringssl-review.googlesource.com/2233
Reviewed-by: Adam Langley <agl@google.com>