app/test: fix integer overflow in memory unit test

memory_autotest loops infinitely when at least one the memsegs
is bigger than 4GB.

The issue is the result of an integer overflow/wraparound of
the offset variable.

Fix it by using the correct type (size_t).

Signed-off-by: Sergio Gonzalez Monroy <sergio.gonzalez.monroy@intel.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>

app/test/test_memory.c

@@ -55,7 +55,8 @@ static int
 test_memory(void)
 {
 	uint64_t s;
-	unsigned i, j;
+	unsigned i;
+	size_t j;
 	const struct rte_memseg *mem;
 
 	/*