Add a libssh2 fuzzer (#2766)

Currently forked off the latest libssh2 code while the changes are
pending in their source tree.

projects/libssh2/Dockerfile

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+# Copyright 2019 Google Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+################################################################################
+
+FROM gcr.io/oss-fuzz-base/base-builder
+MAINTAINER cmeister2@gmail.com
+
+# libssh2 at revision 9662c96 with fuzzer changes on top.
+RUN git clone --depth 1 -b cmeister2/ossfuzz_9662c96 https://github.com/cmeister2/libssh2.git /src/libssh2
+
+WORKDIR $SRC/libssh2
+COPY build.sh $SRC/

projects/libssh2/build.sh

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+#!/bin/bash -eu
+# Copyright 2019 Google Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+################################################################################
+
+# Run the OSS-Fuzz script in the project.
+./tests/ossfuzz/ossfuzz.sh

projects/libssh2/project.yaml

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+homepage: "https://github.com/libssh2/libssh2"
+primary_contact: "will.cosgrove@gmail.com"
+auto_ccs:
+  - "cmeister2@gmail.com"
+sanitizers:
+  - address
+  - undefined