hw/i2c: pmbus: block uninitialised string reads

Devices models calling pmbus_send_string can't be relied upon to
send a non-zero pointer. This logs an error and doesn't segfault.

Reviewed-by: Patrick Venture <venture@google.com>
Signed-off-by: Titus Rwantare <titusr@google.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230322175513.1550412-5-titusr@google.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

hw/i2c/pmbus_device.c

@@ -94,6 +94,13 @@ void pmbus_send64(PMBusDevice *pmdev, uint64_t data)
 
 void pmbus_send_string(PMBusDevice *pmdev, const char *data)
 {
+    if (!data) {
+        qemu_log_mask(LOG_GUEST_ERROR,
+                      "%s: %s: uninitialised read from 0x%02x\n",
+                      __func__, DEVICE(pmdev)->canonical_path, pmdev->code);
+        return;
+    }
+
     size_t len = strlen(data);
     g_assert(len > 0);
     g_assert(len + pmdev->out_buf_len < SMBUS_DATA_MAX_LEN);