ENV/shared: handle untapped e.g. gcc48 formula.

In this case we need to handle the throwing on an exception when
attempting to initialize the gcc48 Formula object.

This initialization should be unnecessary if the core GCC is already
installed and rescued if not.

Library/Homebrew/extend/ENV/shared.rb

@@ -188,16 +188,21 @@ module SharedEnvExtension
   def gcc_version_formula(version)
     gcc_formula = Formulary.factory("gcc")
     gcc_name = 'gcc' + version.delete('.')
-    gcc_versions_formula = Formulary.factory(gcc_name)
 
     if gcc_formula.opt_prefix.exist?
-      gcc_formula
-    elsif gcc_versions_formula.opt_prefix.exist?
+      return gcc_formula
+    end
+
+    gcc_versions_formula = Formulary.factory(gcc_name) rescue nil
+
+    if gcc_versions_formula && gcc_versions_formula.opt_prefix.exist?
       gcc_versions_formula
     elsif gcc_formula.version.to_s.include?(version)
       gcc_formula
-    else
+    elsif gcc_versions_formula
       gcc_versions_formula
+    else
+      Formulary.factory(gcc_name)
     end
   end