gas: ginsn: x86: pacify Wmaybe-uininitialized compiler warning

Fix PR binutils/32091

After commit d56083b5, some gcc versions
may warn about unintialized usage of ginsn_func.  Albeit false positive,
adapt the code to escape the warning.

gas/config/
	* tc-i386-ginsn.c (x86_ginsn_indirect_branch): Early exit if
	unexpected args.