EDAC/versal: Check user-supplied data before injecting an error

The function inject_data_ue_store() lacks a NULL check for the user
passed values. To prevent below kernel crash include a NULL check.

Call trace:

  kstrtoull
  kstrtou8
  inject_data_ue_store
  full_proxy_write
  vfs_write
  ksys_write
  __arm64_sys_write
  invoke_syscall
  el0_svc_common.constprop.0
  do_el0_svc
  el0_svc
  el0t_64_sync_handler
  el0t_64_sync

Fixes: 83bf2405 ("EDAC/versal: Make the bit position of injected errors configurable")
Signed-off-by: Shubhrajyoti Datta <shubhrajyoti.datta@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20240425121942.26378-3-shubhrajyoti.datta@amd.com