Check existence rather than rescue exceptions

Library/Homebrew/formula.rb

@@ -80,7 +80,7 @@ class Formula
 
   # if the dir is there, but it's empty we consider it not installed
   def installed?
-    installed_prefix.children.length > 0 rescue false
+    (dir = installed_prefix).directory? && dir.children.length > 0
   end
 
   def pinable?

Library/Homebrew/test/test_formula.rb

@@ -14,6 +14,22 @@ class FormulaTests < Test::Unit::TestCase
     assert_kind_of Pathname, f.prefix
   end
 
+  def test_installed?
+    f = TestBall.new
+    f.stubs(:installed_prefix).returns(stub(:directory? => false))
+    assert !f.installed?
+
+    f.stubs(:installed_prefix).returns(
+      stub(:directory? => true, :children => [])
+    )
+    assert !f.installed?
+
+    f.stubs(:installed_prefix).returns(
+      stub(:directory? => true, :children => [stub])
+    )
+    assert f.installed?
+  end
+
   def test_class_naming
     assert_equal 'ShellFm', Formula.class_s('shell.fm')
     assert_equal 'Fooxx', Formula.class_s('foo++')