cgen: fix for (int i = 0; i < 10; i++, a++) { (multiple expressions in the inc part)

vlib/v/gen/c/for.v

@@ -70,8 +70,22 @@ fn (mut g Gen) for_c_stmt(node ast.ForCStmt) {
 		}
 		g.write('; ')
 		if node.has_inc {
+			mut processed := false
+			if node.inc is ast.ExprStmt {
+				if node.inc.expr is ast.ConcatExpr {
+					for inc_expr_idx, inc_expr in node.inc.expr.vals {
+						g.expr(inc_expr)
+						if inc_expr_idx < node.inc.expr.vals.len - 1 {
+							g.write(', ')
+						}
+					}
+					processed = true
+				}
+			}
+			if !processed {
 				g.stmt(node.inc)
 			}
+		}
 		g.writeln(') {')
 		g.is_vlines_enabled = true
 		g.inside_for_c_stmt = false

vlib/v/tests/for_c_multiple_expressions_in_increment_test.v

@@ -0,0 +1,6 @@
+fn test_2_expressions_in_same_for_increment_part() {
+	mut a := 0
+	for i := 0; i < 10; i++, a++ {
+		assert a == i
+	}
+}