fix uninitialized field in ada-lang.c (struct match_data)

fix uninitialized field in ada-lang.c (struct match_data)

Field found_sym in add_nonlocal_symbols's struct match_data is
used uninitialized.  Rather than adding the initialization of
this field (to zero), we set the entire structure to zero first,
and then set the fields that need to be initialized to non-zero
next.

gdb/ChangeLog:

        * ada-lang.c (add_nonlocal_symbols): Initialize data to
        all zeros.  Remove setting of data.arg_sym to NULL.