shell: Resist inputs starting with dashes.

shell/caucase.sh

@@ -938,7 +938,7 @@ EOF
           crt="$(getCertificate "${ca_anon_url}/${mode_path}" "$csr_id")"
           status=$?
           test $status -ne 0 && return $status
-          if [ "$crt_path" = '-' ]; then
+          if [ "x$crt_path" = 'x-' ]; then
             printf '%s\n' "$crt"
           else
             if [ -e "$crt_path" ]; then
@@ -1002,7 +1002,7 @@ EOF
           )"
           status=$?
           test $status -ne 0 && return $status
-          if [ "$csr_path" = '-' ]; then
+          if [ "x$csr_path" = 'x-' ]; then
             printf '%s\n' "$csr"
           else
             printf '%s\n' "$csr" > "$csr_path"