nss: improve error handling in Curl_nss_random()

The vtls layer now checks the return value, so it is no longer necessary
to abort if a random number cannot be provided by NSS.  This also fixes
the following Coverity report:

Error: FORWARD_NULL (CWE-476):
lib/vtls/nss.c:1918: var_compare_op: Comparing "data" to null implies that "data" might be null.
lib/vtls/nss.c:1923: var_deref_model: Passing null pointer "data" to "Curl_failf", which dereferences it.
lib/sendf.c:154:3: deref_parm: Directly dereferencing parameter "data".