Handle errors reported in empty files

Some plugins (e.g., flake8-future-import) report errors for empty
files. Those plugins default to reporting the line number as 1 which
caused earlier versions of Flake8 3.0 beta to crash on an IndexError

Closes #157

src/flake8/formatting/base.py

@@ -123,8 +123,9 @@ class BaseFormatter(object):
         :rtype:
             str
         """
-        if not self.options.show_source:
-            return None
+        if not self.options.show_source or error.physical_line is None:
+            return ''
+
         pointer = (' ' * error.column_number) + '^'
         # Physical lines have a newline at the end, no need to add an extra
         # one

src/flake8/processor.py

@@ -250,7 +250,13 @@ class FileProcessor(object):
 
     def line_for(self, line_number):
         """Retrieve the physical line at the specified line number."""
-        return self.lines[line_number - 1]
+        adjusted_line_number = line_number - 1
+        # NOTE(sigmavirus24): Some plugins choose to report errors for empty
+        # files on Line 1. In those casese, we shouldn't bother trying to
+        # retrieve a physical line (since none exist).
+        if 0 <= adjusted_line_number < len(self.lines):
+            return self.lines[adjusted_line_number]
+        return None
 
     def next_line(self):
         """Get the next line from the list."""