libiberty: Fix memory leak in ada_demangle when symbol cannot be demangled.

libiberty: Fix memory leak in ada_demangle when symbol cannot be demangled.

When a symbol cannot be demangled in ada_demangle a new demangled VEC
will be allocated without deleting the demangled VEC already in use.

Running testsuite/test-demangle under valgrind will show the leak for
this entry in testsuite/demangle-expected:

    # Elaborated flag (not demangled)
    --format=gnat
    x_E
    <x_E>

 11 bytes in 1 blocks are definitely lost in loss record 1 of 1
    at 0x4C27BE3: malloc (vg_replace_malloc.c:299)
    by 0x413FE7: xmalloc (xmalloc.c:148)
    by 0x4025EC: ada_demangle (cplus-dem.c:930)
    by 0x402C59: cplus_demangle (cplus-dem.c:892)
    by 0x400FEC: main (test-demangle.c:317)

libiberty/ChangeLog:

	* cplus-dem.c (ada_demangle): Initialize demangled to NULL and
	XDELETEVEC demangled when unknown.