Shuffle external mode: Micro-optimizations relying on our engine

1. Variables are pre-initialized to 0.

2. Order of operations within an expression is specific.  Since this
external mode is bundled, we'll have opportunity to revise it along with
any compiler changes.  Meanwhile, it's valid for older versions as well,
and this dirty optimization provides a ~1% speedup.

run/john.conf

@@ -3978,7 +3978,7 @@ void init()
 	max_length = 10;
 
 	/* Prepare a 12-bit count leading zero table */
-	int i; i = -1;
+	int i; i = 0;
 	while (i++ < 12) {
 		int j; j = 0;
 		while (j < 0x1000)
@@ -4025,7 +4025,7 @@ void next()
 			k = id[i];
 			int mask; mask = prev_unseen;
 			while (k--)
-				mask &= mask - 1;
+				mask = mask & --mask; /* Would be UB in C /
 			/* Count how many total indices we skipped */
 			if ((j = clz[mask & 0xfff]) == 12) {
 				while ((k = clz[(mask >>= 12) & 0xfff]) == 12)