CVE-2026-74327: vmalloc: fix NULL pointer dereference in is_vm_area_hugepages()

In the Linux kernel, the following vulnerability has been resolved:

vmalloc: fix NULL pointer dereference in isvmareahugepages()

findvmarea() can return NULL if the given address is not a valid vmalloc area. Check the return value before dereferencing it to avoid a kernel crash.