hw/ssi/xilinx_spips: Avoid variable length array

In the stripe8() function we use a variable length array; however
we know that the maximum length required is MAX_NUM_BUSSES. Use
a fixed-length array and an assert instead.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Francisco Iglesias <frasse.iglesias@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-id: 20190328152635.2794-1-peter.maydell@linaro.org

hw/ssi/xilinx_spips.c

@@ -429,12 +429,14 @@ static void xlnx_zynqmp_qspips_reset(DeviceState *d)
 
 static inline void stripe8(uint8_t *x, int num, bool dir)
 {
-    uint8_t r[num];
-    memset(r, 0, sizeof(uint8_t) * num);
+    uint8_t r[MAX_NUM_BUSSES];
     int idx[2] = {0, 0};
     int bit[2] = {0, 7};
     int d = dir;
 
+    assert(num <= MAX_NUM_BUSSES);
+    memset(r, 0, sizeof(uint8_t) * num);
+
     for (idx[0] = 0; idx[0] < num; ++idx[0]) {
         for (bit[0] = 7; bit[0] >= 0; bit[0]--) {
             r[idx[!d]] |= x[idx[d]] & 1 << bit[d] ? 1 << bit[!d] : 0;