netdev-dpdk: Avoid undefined behavior processing devargs

netdev-dpdk: Avoid undefined behavior processing devargs

In "Use of library functions" in the C standard, the following statement
is written to apply to all library functions:

  If an argument to a function has an invalid value (such as ... a
  null pointer ... the behavior is undefined.

Later, under the "String handling" section, "Comparison functions" no
exception is listed for strcmp, which means NULL is invalid.  It may
be possible for the smap_get to return NULL.

Given the above, we must check that new_devargs is not null.  The check
against NULL for new_devargs later in the function is still valid.

Fixes: 55e075e65ef9 ("netdev-dpdk: Arbitrary 'dpdk' port naming")
Signed-off-by: Aaron Conole <aconole@redhat.com>
Acked-by: Ciara Loftus <ciara.loftus@intel.com>
Signed-off-by: Ian Stokes <ian.stokes@intel.com>