Fix possibly unitialized variables in __cxa_demangle_gnu3()

After 0ee0dbfb0d26cf4bc37f24f12e76c7f532b0f368 where I imported a more
recent libcxxrt snapshot, the variables 'rtn' and 'has_ret' could in
some cases be used while still uninitialized. Most obviously this would
lead to a jemalloc complaint about a bad free(), aborting the program.

Fix this by initializing a bunch variables in their declarations. This
change has also been sent upstream, with some additional changes to be
used in their testing framework.

PR: 253226
MFC after: 3 days