Fix edge case in switch with single else

ir_gen_switch_expr doesn't set the switch_br field at all if there are
zero cases, detect this situation and handle it gracefully.

Closes #4322

src/ir.cpp

@@ -22768,7 +22768,9 @@ static IrInstGen *ir_analyze_instruction_switch_else_var(IrAnalyze *ira,
         }
         // Make note of the errors handled by other cases
         ErrorTableEntry **errors = allocate<ErrorTableEntry *>(ira->codegen->errors_by_index.length);
-        for (size_t case_i = 0; case_i < instruction->switch_br->case_count; case_i += 1) {
+        // We may not have any case in the switch if this is a lone else
+        const size_t switch_cases = instruction->switch_br ? instruction->switch_br->case_count : 0;
+        for (size_t case_i = 0; case_i < switch_cases; case_i += 1) {
             IrInstSrcSwitchBrCase *br_case = &instruction->switch_br->cases[case_i];
             IrInstGen *case_expr = br_case->value->child;
             if (case_expr->value->type->id == ZigTypeIdErrorSet) {

test/stage1/behavior/switch.zig

@@ -479,3 +479,17 @@ test "switch on pointer type" {
     comptime expect(2 == S.doTheTest(S.P2));
     comptime expect(3 == S.doTheTest(S.P3));
 }
+
+test "switch on error set with single else" {
+    const S = struct {
+        fn doTheTest() void {
+            var some: error{Foo} = error.Foo;
+            expect(switch (some) {
+                else => |a| true,
+            });
+        }
+    };
+
+    S.doTheTest();
+    comptime S.doTheTest();
+}