cgen: fix the C codegen error for '_ = map[key]' (fix #11999) (#12000)

vlib/v/gen/c/cgen.v

@@ -2925,6 +2925,9 @@ fn (mut g Gen) gen_assign_stmt(assign_stmt ast.AssignStmt) {
 			g.right_is_opt = true
 		}
 		if blank_assign {
+			if val is ast.IndexExpr {
+				g.assign_op = .decl_assign
+			}
 			if is_call {
 				old_is_void_expr_stmt := g.is_void_expr_stmt
 				g.is_void_expr_stmt = true

vlib/v/tests/map_get_assign_blank_test.v

@@ -1,6 +1,6 @@
 type Abc = int | string
 
-fn test_map_get_assign_blank() {
+fn test_map_get_decl_assign_blank() {
 	x := map[string]Abc{}
 	_ := x['nonexisting']
 	if y := x['nonexisting'] {
@@ -8,3 +8,12 @@ fn test_map_get_assign_blank() {
 	}
 	assert true
 }
+
+fn test_map_get_assign_blank() {
+	x := map[string]Abc{}
+	_ = x['nonexisting']
+	if y := x['nonexisting'] {
+		println(y)
+	}
+	assert true
+}