Fix null pointer dereference in yasm_section_bcs_append (yasm#263)

Author: haruki3hhh

libyasm/section.c

@@ -629,6 +629,10 @@ yasm_bytecode *
 yasm_section_bcs_append(yasm_section *sect, yasm_bytecode *bc)
 {
     if (bc) {
+        if (!sect) {
+            yasm_error_set(YASM_ERROR_VALUE, "Attempt to append bytecode to a NULL section or with a NULL bytecode");
+            return NULL;
+        }
         if (bc->callback) {
             bc->section = sect;     /* record parent section */
             STAILQ_INSERT_TAIL(&sect->bcs, bc, link);