[opam] do not crash when the switch does not exist

Summary: Fixes #1258.

Reviewed By: martintrojer

Differential Revision: D21548073

fbshipit-source-id: ddb994910

scripts/opam_utils.sh

@@ -58,8 +58,11 @@ opam_switch_create_if_needed () {
             break
         fi
     done
+    opam_root="${OPAMROOT:-}"
     if [ "$switch_exists" = "no" ]; then
-        rm -rf "$OPAMROOT/$switch"
+        if [ -n "$opam_root" ]; then
+            rm -rf "$opam_root/$switch" || true
+        fi
         opam switch create "$switch" "$compiler"
     fi
 }