Skip empty write buffers when unserializing parser

At commit 8490a7ac the following `fuzzing/usbredirparserfuzz` input causes
the instantiation of empty write buffers:

$ base64 -d <<'EOF' > testcase6474540506021888
QAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAYAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAQAAAAG
AAAAAN7/AAAACAA=
EOF

Empty write buffers trigger all kinds of issues, in part because they cause
calls to write(2) with a zero length.

Signed-off-by: Michael Hanselmann <public@hansmi.ch>