of: module: prevent NULL pointer dereference in vsnprintf()

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

of: module: prevent NULL pointer dereference in vsnprintf()

In of_modalias(), we can get passed the str and len parameters which would

cause a kernel oops in vsnprintf() since it only allows passing a NULL ptr

when the length is also 0. Also, we need to filter out the negative values

of the len parameter as these will result in a really huge buffer since

snprintf() takes size_t parameter while ours is ssize_t...

Found by Linux Verification Center (linuxtesting.org) with the Svace static

analysis tool.