Fix IndexError when parsing GYP files.

GYP automatically turns variables ending in _dir, _file or _path into
absolute paths but didn't check for empty strings.

It interacted badly with variables inherited through the environment
from npm, the `scripts-prepend-node-path=false` setting in particular
because it is turned into `npm_config_script_prepend_node_path=`.

Fixes: nodejs#1217
PR-URL: nodejs#1267
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>

gyp/pylib/gyp/input.py

@@ -2033,7 +2033,7 @@ def MakePathRelative(to_file, fro_file, item):
         gyp.common.RelativePath(os.path.dirname(fro_file),
                                 os.path.dirname(to_file)),
                                 item)).replace('\\', '/')
-    if item[-1] == '/':
+    if item[-1:] == '/':
       ret += '/'
     return ret