Test length of comps when listing mounts

If the stdout returned from the mount command ends with a newline char,
this produces an empty string during splitting.  This empty string is
attempted to be ``split()`` on whitespace, and thus produces an
empty-length list, which errors on the following line because no
elements are present.

salt/modules/mount.py

@@ -41,7 +41,8 @@ def _list_mounts():
 
     for line in mounts.split('\n'):
         comps = re.sub(r"\s+", " ", line).split()
-        ret[comps[2]] = comps[0]
+        if len(comps) >= 3:
+            ret[comps[2]] = comps[0]
     return ret