ffs: handle 0 argument (bug #8968)

Handle an argument of 0 in ffs(), instead of looping indefinitely.
Add an ffs prototype to dix.h, and add includes to ffs.c.
(cherry picked from 34164e551e4c3909322d50b09835ca4ac1d49d68 commit)

dix/ffs.c

@@ -36,6 +36,8 @@ int
 ffs(int i)
 {
     int j;
+    if (i == 0)
+        return 0;
     for (j = 1; (i & 1) == 0; j++)
 	i >>= 1;
     return j;

include/dix.h

@@ -817,4 +817,7 @@ typedef struct {
 extern int xstrcasecmp(char *s1, char *s2);
 #endif
 
+/* ffs.c */
+extern int ffs(int i);
+
 #endif /* DIX_H */