in case the length is 0 don't decrement the lenght

git-svn-id: svn://svn.lighttpd.net/lighttpd/branches/lighttpd-merge-1.4.x@877 152afb58-edef-0310-8abb-c4023f1b3aa9

src/buffer.h

@@ -119,8 +119,8 @@ int light_isalnum(int c);
 #define BUFFER_APPEND_SLASH(x) \
 	if (x->used > 1 && x->ptr[x->used - 2] != '/') { BUFFER_APPEND_STRING_CONST(x, "/"); }
 
-#define CONST_STR_LEN(x) x, sizeof(x) - 1
-#define CONST_BUF_LEN(x) x->ptr, x->used - 1
+#define CONST_STR_LEN(x) x, x ? sizeof(x) - 1 : 0
+#define CONST_BUF_LEN(x) x->ptr, x->used ? x->used - 1 : 0
 
 
 #define SEGFAULT() do { fprintf(stderr, "%s.%d: aborted\n", __FILE__, __LINE__); abort(); } while(0)