SF bug #973092:  inspect.getframeinfo bug if 'context' is to big

Make sure the start argument is not negative.
diff --git a/Lib/inspect.py b/Lib/inspect.py
index 8a58ce9..12c9cb7 100644
--- a/Lib/inspect.py
+++ b/Lib/inspect.py
@@ -756,7 +756,7 @@
             lines = index = None
         else:
             start = max(start, 1)
-            start = min(start, len(lines) - context)
+            start = max(0, min(start, len(lines) - context))
             lines = lines[start:start+context]
             index = lineno - 1 - start
     else: