Don't store incorrect values in zones_map backport

[#4942 state:committed]
Signed-off-by: NJosé Valim <jose.valim@gmail.com>

activesupport/lib/active_support/values/time_zone.rb

@@ -387,7 +387,9 @@ def zones_map
       def [](arg)
         case arg
           when String
-            zones_map[arg] ||= lookup(arg)
+            if tz = lookup(arg)
+              zones_map[arg] ||= tz
+            end
           when Numeric, ActiveSupport::Duration
             arg *= 3600 if arg.abs <= 13
             all.find { |z| z.utc_offset == arg.to_i }