interp: don't crash on declaring empty names

Updates mvdan#332.

interp/interp.go

@@ -876,6 +876,11 @@ func (r *Runner) cmd(ctx context.Context, cm syntax.Command) {
 		for _, as := range x.Assigns {
 			for _, as := range r.flattenAssign(as) {
 				name := as.Name.Value
+				if !syntax.ValidName(name) {
+					r.errf("declare: invalid name %q\n", name)
+					r.exit = 1
+					return
+				}
 				vr := r.lookupVar(as.Name.Value)
 				vr.Value = r.assignVal(as, valType)
 				if global {

interp/interp_test.go

@@ -1869,6 +1869,8 @@ set +o pipefail
 	{"a=x=y; declare $a; echo $a $x", "x=y y\n"},
 	{"a='x=(y)'; declare $a; echo $a $x", "x=(y) (y)\n"},
 	{"a='x=b y=c'; declare $a; echo $x $y", "b c\n"},
+	{"declare =bar", "declare: invalid name \"=bar\"\nexit status 1 #JUSTERR"},
+	{"declare $unset=$unset", "declare: invalid name \"\"\nexit status 1 #JUSTERR"},
 
 	// export
 	{"declare foo=bar; env | grep '^foo='", "exit status 1"},