Fixed null reference exception generated when Mono's DefaultCLR registry value was missing.

git-svn-id: svn://svn.sharpdevelop.net/sharpdevelop/branches/2.0@1409 1ccf3a8d-04fe-1044-b7c0-cef0b8235c61

src/Libraries/ICSharpCode.Build.Tasks/Project/MonoToolLocationHelper.cs

@@ -172,7 +172,9 @@ namespace ICSharpCode.Build.Tasks
 				if (key != null) {
 					string readValue = (string)key.GetValue(name);
 					key.Close();
-					return readValue;
+					if (readValue != null) {
+						return readValue;
+					}
 				}
 			} catch (SecurityException) { }