simplify cygdb test code to remove an unnecessary indentation level

Cython/Debugger/Tests/TestLibCython.py

@@ -35,7 +35,9 @@ build_ext = sys.modules['Cython.Distutils.build_ext']
 have_gdb = None
 def test_gdb():
     global have_gdb
-    if have_gdb is None:
+    if have_gdb is not None:
+        return have_gdb
+
     try:
         p = subprocess.Popen(['gdb', '-v'], stdout=subprocess.PIPE)
         have_gdb = True