Fix corner case of empty input in tree fragment processing: min() fails on empty sequence.

Cython/Compiler/TreeFragment.py

@@ -209,6 +209,7 @@ def strip_common_indent(lines):
     """Strips empty lines and common indentation from the list of strings given in lines"""
     # TODO: Facilitate textwrap.indent instead
     lines = [x for x in lines if x.strip() != u""]
+    if lines:
         minindent = min([len(_match_indent(x).group(0)) for x in lines])
         lines = [x[minindent:] for x in lines]
     return lines