HTTP2: Fix double free

Found by Clang Analyzer

Backport from https://github.com/tatsuhiro-t/nghttp2/commit/62423f5949001c375c8e325296c74c92d1be0965
Signed-off-by: Tatsuhiro Tsujikawa <tatsuhiro.t@gmail.com>

Change-Id: Ic70f74b55ab80ca0b8d5042f07a29c9d14af4fe2
Reviewed-on: https://code.wireshark.org/review/3059
Reviewed-by: Evan Huus <eapache@gmail.com>