Remove trailing slash from $SRCDIR and check for required env vars

If $SRCDIR has a trailing slash (e.g., `kernel/`), the command
`cp -r "$SRCDIR" "${SRCDIR}.orig"` incorrectly copies the backup
*inside* the source directory instead of alongside it. This causes
the subsequent `diff -Nupr` to fail or compute an incorrect diff.

Also checks for the definitions of required env vars.

Author: Tomcat-42

test/integration/rebase-patches

@@ -18,7 +18,13 @@
 #    ID=rhel VERSION_ID=7.8 ./rebase-patches rhel-7.7/*{.patch,.disabled}
 # % cp rhel-7.7/*.test rhel-7.8/
 
-OUTDIR=$(pwd)/${ID}-${VERSION_ID}
+if [ -z "$SRCDIR" ] || [ -z "$ID" ] || [ -z "$VERSION_ID" ]; then
+  echo "ERROR: SRCDIR, ID, and VERSION_ID environment variables must be set"
+  exit 1
+fi
+
+SRCDIR="${SRCDIR%/}"
+OUTDIR="$(pwd)/${ID}-${VERSION_ID}"
 mkdir -p "$OUTDIR"
 
 echo "* Making backup copy of kernel sources"