sv: fix up end label checking

- disallow [gen]blocks with an end label but not begin label
- check validity of module end label
- fix memory leak of package name and end label
- fix memory leak of module end label

tests/simple/matching_end_labels.sv

@@ -0,0 +1,29 @@
+module top(
+    output reg [7:0]
+    out1, out2, out3, out4
+);
+    initial begin
+        begin : blk1
+            reg x;
+            x = 1;
+        end
+        out1 = blk1.x;
+        begin : blk2
+            reg x;
+            x = 2;
+        end : blk2
+        out2 = blk2.x;
+    end
+    if (1) begin
+        if (1) begin : blk3
+            reg x;
+            assign x = 3;
+        end
+        assign out3 = blk3.x;
+        if (1) begin : blk4
+            reg x;
+            assign x = 4;
+        end : blk4
+        assign out4 = blk4.x;
+    end
+endmodule