Delete temporary string within demangler even in failure cases.

A call to demangle_template might allocate storage within a temporary
string even if the call to demangle_template eventually returns
failure.

This will never cause the demangler to crash, but does leak memory, as
a result I've not added any tests for this.

Calling string_delete is safe, even if nothing is allocated into the
string, the string is initialised with string_init, so we know the
internal pointers are NULL.

libiberty/ChangeLog

	* cplus-dem.c (do_type): Call string_delete even if the call to
	demangle_template fails.