In replace_operator_with_call, we resize the elts array like this:
...
  exp->nelts = exp->nelts + 7 - oplen;
  exp->resize (exp->nelts);
...

Although all the current callers ensure that the new size is bigger, it could
also be smaller, in which case the following memmove possibly reads out of
bounds:
...
   memmove (exp->elts + pc + 7, exp->elts + pc + oplen,
           EXP_ELEM_TO_BYTES (save_nelts - pc - oplen));
...

Fix this by doing the resize after the memmove in case the new size is
smaller.

Tested on x86_64-linux.

gdb/ChangeLog:

2020-12-07  Tom de Vries  <tdevries@suse.de>

	* ada-lang.c (replace_operator_with_call): Handle shrink resize.