d: fix ASAN in option processing

Fixes:

==129444==ERROR: AddressSanitizer: global-buffer-overflow on address 0x00000666ca5c at pc 0x000000ef094b bp 0x7fffffff8180 sp 0x7fffffff8178
READ of size 4 at 0x00000666ca5c thread T0
    #0 0xef094a in parse_optimize_options ../../gcc/d/d-attribs.cc:855
    #1 0xef0d36 in d_handle_optimize_attribute ../../gcc/d/d-attribs.cc:916
    #2 0xef107e in d_handle_optimize_attribute ../../gcc/d/d-attribs.cc:887
    #3 0xff85b1 in decl_attributes(tree_node**, tree_node*, int, tree_node*) ../../gcc/attribs.c:829
    #4 0xef2a91 in apply_user_attributes(Dsymbol*, tree_node*) ../../gcc/d/d-attribs.cc:427
    #5 0xf7b7f3 in get_symbol_decl(Declaration*) ../../gcc/d/decl.cc:1346
    #6 0xf87bc7 in get_symbol_decl(Declaration*) ../../gcc/d/decl.cc:967
    #7 0xf87bc7 in DeclVisitor::visit(FuncDeclaration*) ../../gcc/d/decl.cc:808
    #8 0xf83db5 in DeclVisitor::build_dsymbol(Dsymbol*) ../../gcc/d/decl.cc:146

for the following test-case: gcc/testsuite/gdc.dg/attr_optimize1.d.

gcc/d/ChangeLog:

	* d-attribs.cc (parse_optimize_options): Check index before
	accessing cl_options.