only check a library folder if it actually exists

otherwise this breaks for no reason if /usr/local/lib is missing.

port/PyAssimp/pyassimp/helper.py

@@ -205,6 +205,7 @@ def search_library():
     candidates = []
     # test every file
     for curfolder in [folder]+additional_dirs:
+        if os.path.isdir(curfolder):
             for filename in os.listdir(curfolder):
                 # our minimum requirement for candidates is that
                 # they should contain 'assimp' somewhere in