Only modify autoindex.rst if it exists

This fixes doc build errors when using older Sphinx versions.

Change-Id: I0ed9bed311d6309257c0a111671d0fea46f5da7c
Closes-bug: #1387343

doc/source/conf.py

@@ -41,13 +41,15 @@ exclude_patterns = [
 ]
 
 # Prune the excluded patterns from the autoindex
-for line in fileinput.input('api/autoindex.rst', inplace=True):
-    found = False
-    for pattern in exclude_patterns:
-        if fnmatch.fnmatch(line, '*' + pattern[4:]):
-            found = True
-    if not found:
-        print line,
+PATH = 'api/autoindex.rst'
+if os.path.isfile(PATH) and os.access(PATH, os.R_OK):
+    for line in fileinput.input(PATH, inplace=True):
+        found = False
+        for pattern in exclude_patterns:
+            if fnmatch.fnmatch(line, '*' + pattern[4:]):
+                found = True
+        if not found:
+            print line,
 
 # The suffix of source filenames.
 source_suffix = '.rst'
@@ -91,4 +93,4 @@ latex_documents = [
 ]
 
 # Example configuration for intersphinx: refer to the Python standard library.
-#intersphinx_mapping = {'http://docs.python.org/': None}
+#intersphinx_mapping = {'http://docs.python.org/': None}