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

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.

Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/1d211023-3923-685b-20f0-f3f90ea56e1f@omp.ru
Signed-off-by: Rob Herring <robh@kernel.org>