Form.cc: fix radiobutton reporting wrong state

When a radiobutton (belonging to a normal radiobutton group)
has a /V key matching his 'OnStr' state, then when you
ask that radiobutton for his state (eg. radiobutton->state())
it will wrongly return 'true', when really the active
radiobutton is another one in the group.

This happens because the faulty radiobutton was not passing
the getState() call to his Parent (which every radiobutton
in a group should do, as the Parent stores the value of the current
active item).

The code was not doing it because it had a valid AppearanceState
(/V key). That behaviour may be right for checkboxes but not for
radiobuttons.

A testcase is included. An example definition of an
affected radiobutton follows:

/F 4
/FT /Btn
/Ff 49152
/AP /N /Beer 59 0 R /Off 61 0 R
/AS /Beer
/MK /BC [1,0,0] /BG [1,1,1] /CA (4)
/P 20 0 R
/Parent 8 0 R
/Rect [235.277,654.247,249.224,668.194]
/Subtype /Widget
/Type /Annot
/V /Beer

Fixes issue #159
6 jobs from BUG_tcpdf_checkbox in 14 minutes (queued for 1 second)
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passed #100757
build

00:04:44

passed #100762
build_android

00:01:34

passed #100758
build_clang_libcpp

00:03:58

passed #100761
build_clazy

00:04:21

passed #100759
build_gcc_49

00:05:35

passed #100760
build_mingw64_fedora

00:14:00