Fix doctor hanging if xcode-select is bad

Also fix typo in the superenv DEVELOPER_DIR set detection code.

Library/Homebrew/cmd/doctor.rb

@@ -942,6 +942,7 @@ def check_os_version
 end
 
   def check_xcode_license_approved
+    return if MacOS::Xcode.bad_xcode_select_path?
     # If the user installs Xcode-only, they have to approve the
     # license or no "xc*" tool will work.
     <<-EOS.undent if `/usr/bin/xcrun clang 2>&1` =~ /license/ and not $?.success?

Library/Homebrew/superenv.rb

@@ -184,7 +184,7 @@ class << ENV
     # nothing is valid, it still fixes most usage to supply a valid path that
     # is not "/".
     if MacOS::Xcode.bad_xcode_select_path?
-      (Xcode.prefix || HOMEBREW_PREFIX).to_s
+      (MacOS::Xcode.prefix || HOMEBREW_PREFIX).to_s
     end
   end