try disabling IM for oss-fuzz

libvips has to use ReadImage() when it opens an image via MagickCore,
and this can allocate very large amounts of memory, killing the fuzzer.

IM is already part of oss-fuzz, so we are not gaining much by
replicating that work. This patch attempts to disable IM for fuzzing tests.

.travis.yml

@@ -8,6 +8,7 @@ before_script:
     --disable-dependency-tracking
     --with-jpeg-includes=$JPEG/include
     --with-jpeg-libraries=$JPEG/lib
+    --with-magick=$WITH_MAGICK
   - make -j$JOBS -s 
 script:
   - make -j$JOBS -s -k V=0 VERBOSE=1 check
@@ -30,6 +31,7 @@ matrix:
         - PYVIPS_VERSION=master
         - JPEG=/usr
         - JOBS=`nproc`
+        - WITH_MAGICK=yes
       cache: ccache
 
     - os: linux
@@ -41,6 +43,7 @@ matrix:
         - PYVIPS_VERSION=master
         - JPEG=/usr
         - JOBS=`nproc`
+        - WITH_MAGICK=no
         - CFLAGS="-fsanitize=address,undefined -fno-omit-frame-pointer -fopenmp -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION"
         - LDFLAGS="-fsanitize=address,undefined -dynamic-asan -fopenmp=libiomp5"
         - ASAN_DSO=/usr/local/clang-7.0.0/lib/clang/7.0.0/lib/linux/libclang_rt.asan-x86_64.so
@@ -66,6 +69,7 @@ matrix:
         - PYVIPS_VERSION=master
         - JPEG=/usr/local
         - JOBS="`sysctl -n hw.ncpu`"
+        - WITH_MAGICK=yes
         - PATH="/usr/local/opt/ccache/libexec:$PATH"
         - HOMEBREW_NO_AUTO_UPDATE=1
       cache: ccache