fix(parser): Support default button index for actions
fix(parser): Support default button index for actions

Defining an action without a button index caused a segfault since
the parser was treating it as the close of a previously opened action
block. This makes the parser use button LEFT as a fallback.

Also ensure that an action block has been opened before
attempting to close the block.

Fixes jaagr/lemonbuddy#104

jaagr/lemonbuddy,jaagr/lemonbuddy,jaagr/lemonbuddy,jaagr/polybar,jaagr/polybar,jaagr/polybar