x86: Check unbalanced braces in memory reference

Check unbalanced braces in memory reference to avoid assembler crash
caused by

commit e87fb6a6
Author: Jan Beulich <jbeulich@suse.com>
Date:   Wed Oct 5 09:16:24 2022 +0200

    x86/gas: support quoted address scale factor in AT&T syntax

	PR gas/30248
	* config/tc-i386.c (i386_att_operand): Check unbalanced braces
	in memory reference.
	* testsuite/gas/i386/i386.exp: Run pr30248.
	* testsuite/gas/i386/pr30248.d: New file.
	* testsuite/gas/i386/pr30248.err: Likewise.
	* testsuite/gas/i386/pr30248.s: Likewise.