[LibraryRepresentation] Fix crash in #prefix_header_path

lib/cocoapods/hooks/library_representation.rb

@@ -27,9 +27,9 @@ module Pod
       # @return [Pathname] The path of the prefix_header
       #
       def prefix_header_path
-        library.prefix_header_path
         UI.warn "LibraryRepresentation#prefix_header_path is deprecated. " \
           "Use the specification `prefix_header_contents` attribute."
+        library.prefix_header_path
       end
       alias :prefix_header_filename :prefix_header_path