[nautilus] eel-string: don't segfault in eel_str_replace_substring for NULL strings

[Cosimo Cecchi <cosimoc src gnome org>]

commit 45ee835907056a95c4ae9dad5f7913620ed219d7
Author: Cosimo Cecchi <cosimoc gnome org>
Date:   Tue Oct 25 12:18:31 2011 -0400

    eel-string: don't segfault in eel_str_replace_substring for NULL strings
    
    Just check if the strings are != NULL before calling strlen() on them.

 eel/eel-string.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/eel/eel-string.c b/eel/eel-string.c
index 27c6f4e..3d6f9d3 100644
--- a/eel/eel-string.c
+++ b/eel/eel-string.c
@@ -206,8 +206,8 @@ eel_str_replace_substring (const char *string,
 		return NULL;
 	}
 
-	substring_length = strlen (substring);
-	replacement_length = strlen (replacement);
+	substring_length = substring ? strlen (substring) : 0;
+	replacement_length = replacement ? strlen (replacement) : 0;
 
 	result_length = strlen (string);
 	for (p = string; ; p = substring_position + substring_length) {