fix(pixels): initialise all members of strands array

[tyeth]

Fixes #926

Previously the strands array was only initialising the first member with the supplied initialiser (and the remainder zero initialised rather than using the supplied values):

strand_s strands[MAX_PIXEL_STRANDS]{
    nullptr,
    nullptr,
    wippersnapper_pixels_v1_PixelsType_PIXELS_TYPE_UNSPECIFIED,
    0,
    0,
    wippersnapper_pixels_v1_PixelsOrder_PIXELS_ORDER_UNSPECIFIED,
    -1,
    -1,
    -1};

This meant the safety check of getStrandIndex(atoi(pin)) did a check on the first element and was fine, but would presumably crash because the pins being checked were not -1 when iterating over further strands array elements, allowing the write to proceed on an unsetup PWM channel.

Change is to do the default intialising of all members in a new constructor.

[tyeth]

I'll test with protomq against v1 protos and send a write without an add. Should crash on released firmware and not on this.