Fix rare case of wmem map being O(n)

For 'x' equal to 0, HASH() macro also returns 0 which makes wmem map O(n).
When random generator will return 0 just use 1.

Change-Id: If484091352a719aea27135a705d37ff4c184a13b
Reviewed-on: https://code.wireshark.org/review/1387
Reviewed-by: Michael Mann <mmann78@netscape.net>

epan/wmem/wmem_map.c

@@ -36,6 +36,8 @@ void
 wmem_init_hashing(void)
 {
     x = g_random_int();
+    if (G_UNLIKELY(x == 0))
+        x = 1;
 
     preseed  = g_random_int();
     postseed = g_random_int();