fix dctx initialization within ZSTD_decompress in stack mode

when ZSTD_HEAPMODE=0 (which is not default).

Also : added an associated test (test-fuzzer-stackmode)
run on travis CI

fix #1186

.travis.yml

@@ -20,14 +20,18 @@ matrix:
     - env: Cmd='make clang38install && CC=clang-3.8 make clean msan-fuzztest'
     - env: Cmd='make clang38install && CC=clang-3.8 make clean tsan-test-zstream'
 
+    - env: Cmd='make -C tests test-fuzzer-stackmode'
+
     - env: Cmd='make valgrindinstall && make -C tests clean valgrindTest'
 
     - env: Cmd='make arminstall && make armfuzz'
+
 # Following test is disabled, as there is a bug in Travis' ld
 # preventing aarch64 compilation to complete.
 # > collect2: error: ld terminated with signal 11 [Segmentation fault], core dumped
 # to be re-enabled in a few commit, as it's possible that a random code change circumvent the ld bug
 #    - env: Cmd='make arminstall && make aarch64fuzz'
+
     - env: Cmd='make ppcinstall && make ppcfuzz'
     - env: Cmd='make ppcinstall && make ppc64fuzz'
     - env: Cmd='make -j uasanregressiontest && make clean && make -j msanregressiontest'
@@ -35,6 +39,7 @@ matrix:
     - env: Cmd='make lz4install && make -C tests test-lz4'
 
     - env: Cmd='bash tests/libzstd_partial_builds.sh'
+
     # tag-specific test
     - if: tag =~ ^v[0-9]\.[0-9]
       env: Cmd='make -C tests checkTag && tests/checkTag $TRAVIS_BRANCH'