[python] thinko in previous patch

[Tom Tromey]

In the previous patch I neglected to account for the case where
gdb_datadir can be NULL.  Oops, fixed as appended.

Tom

b/gdb/ChangeLog:
2009-03-03  Tom Tromey  <tromey@redhat.com>

	* python/python.c (gdbpy_new_objfile): Check gdb_datadir.

diff --git a/gdb/python/python.c b/gdb/python/python.c
index b3c3123..c338009 100644
--- a/gdb/python/python.c
+++ b/gdb/python/python.c
@@ -1004,7 +1004,7 @@ gdbpy_new_objfile (struct objfile *objfile)
       input = fopen (debugfile, "r");
     }
 
-  if (!input)
+  if (!input && gdb_datadir)
     {
       /* Also try the same file in a subdirectory of gdb's data
 	 directory.  */