py: Explicitly set uninitialised struct member to false.

Uninitialised struct members get a default value of 0/false, so this is
not strictly needed.  But it actually decreases code size because when
all members are initialised the compiler doesn't need to insert a call
to memset to clear everything.  In other words, setting 1 extra member
to 0 uses less code than calling memset.

ROM savings in bytes: 32-bit unix: 100; bare-arm: 44; stmhal: 52.