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/verilog/module_end_label.ys

@@ -0,0 +1,15 @@
+logger -expect-no-warnings
+read_verilog -sv <<EOF
+module correct_name;
+localparam X = 1;
+endmodule : correct_name
+EOF
+
+design -reset
+
+logger -expect error "Module name \(correct_name\) and end label \(incorrect_name\) don't match\." 1
+read_verilog -sv <<EOF
+module correct_name;
+localparam X = 1;
+endmodule : incorrect_name
+EOF