Remove alloca(0) calls

PR gdb/24653 points out that a gcc snapshot will complain about the
calls to alloca(0) in gdb.

These calls are only needed when using the C alloca.  I'm inclined to
think that no current platform needs this, so this patch removes the
calls.

Let me know what you think.

gdb/ChangeLog
2019-06-14  Tom Tromey  <tromey@adacore.com>

	PR gdb/24653:
	* regcache.c (registers_changed): Don't call alloca.
	* top.c (execute_command): Don't call alloca.