Opcode: fix 'null argument' warning

Opcode: fix 'null argument' warning

HvNAME_get() can return NULL, and strNE() wants non-null args.

[
As a side note: on debugging builds this line

    if (strNE(HvNAME_get(hv),"main")) {

macro-expands into an 18,000 character line (!) due to the fact that
HvNAME_get() is quite a big expansion under debugging, and strNE expands to
strlen, which under gcc expands to a huge macro (which is mainly lots of
different compile-time alternatives depending on which of its args are
constants), that references its args several times.
]