input/vcd: fix parse_header() return value check.

input/vcd: fix parse_header() return value check.

Mixing tests for both a boolean and an SR_ERR at the same time is not
really a good idea.
parse_header() actually returns a boolean so only check if it returns FALSE.

This fixes the following gcc-5 warning:

src/input/vcd.c: In function 'receive':
src/input/vcd.c:506:34: warning: logical not is only applied to the left hand side of comparison [-Wlogical-not-parentheses]
   if (!parse_header(in, in->buf) != SR_OK)
                                  ^