vmtests: harden fetching kernel sources

Ensure that corrupted tar archive won't screw up build.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>

travis-ci/vmtest/checkout_latest_kernel.sh

@@ -17,8 +17,7 @@ if [ ! -d "${REPO_PATH}" ]; then
 	mkdir -p $(dirname "${REPO_PATH}")
 	cd $(dirname "${REPO_PATH}")
 	# attempt to fetch desired bpf-next repo snapshot
-	if wget ${SNAPSHOT_URL} ; then
-		tar xf bpf-next-${LINUX_SHA}.tar.gz
+	if wget ${SNAPSHOT_URL} && tar xf bpf-next-${LINUX_SHA}.tar.gz ; then
 		mv bpf-next-${LINUX_SHA} $(basename ${REPO_PATH})
 	else
 		# but fallback to git fetch approach if that fails