Merged in x_qin/nuttx/null_check_for_open_and_write (pull request #498)

fs/vfs:null check for path on open and buf on write

Null path check is depend on CONFIG_DEBUG_FEATURES and CONFIG_DEBUG_ASSERTIONS, added null checking so it's always performed
Added null checking on buf for write()

Approved-by: Gregory Nutt <gnutt@nuttx.org>