cgen: fix assign map get to blank (fix #11508) (#11982)

Author: yuyi98

vlib/v/gen/c/cgen.v

@@ -2748,6 +2748,7 @@ fn (mut g Gen) gen_assign_stmt(assign_stmt ast.AssignStmt) {
 				g.expr(val)
 				g.writeln(';}')
 			}
+			g.is_assign_lhs = false
 		} else if assign_stmt.op == .assign
 			&& (is_fixed_array_init || (right_sym.kind == .array_fixed && val is ast.Ident)) {
 			mut v_var := ''

vlib/v/tests/map_get_assign_blank_test.v

@@ -0,0 +1,10 @@
+type Abc = int | string
+
+fn test_map_get_assign_blank() {
+	x := map[string]Abc{}
+	_ := x['nonexisting']
+	if y := x['nonexisting'] {
+		println(y)
+	}
+	assert true
+}