patch 9.1.0717: Unnecessary nextcmd NULL checks in parse_command_modifiers()

patch 9.1.0717: Unnecessary nextcmd NULL checks in parse_command_modifiers()

Problem:  Unnecessary nextcmd NULL checks in parse_command_modifiers().
Solution: Remove them (zeertzjq)

Every place parse_command_modifiers() is called, nextcmd is NULL, and
after it's set to non-NULL the function returns very soon.
Even if one day nextcmd may be non-NULL, the NULL checks may still be
wrong as the correct behavior may be overriding nextcmd.

closes: #15620

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>