Do not call memcpy with NULL src pointer

This fix the following ubsan error
check-62.c:36:2: runtime error: null pointer passed as argument 2, which is declared to never be null

skeletons/xer_encoder.c

@@ -80,7 +80,9 @@ xer__buffer_append(const void *buffer, size_t size, void *app_key) {
         size_t new_size = 2 * (xb->allocated_size ? xb->allocated_size : 64);
         char *new_buf = MALLOC(new_size);
         if(!new_buf) return -1;
-        memcpy(new_buf, xb->buffer, xb->buffer_size);
+        if (xb->buffer) {
+            memcpy(new_buf, xb->buffer, xb->buffer_size);
+        }
         FREEMEM(xb->buffer);
         xb->buffer = new_buf;
         xb->allocated_size = new_size;

tests/tests-c-compiler/check-src/check-60.c

@@ -17,7 +17,9 @@ _buf_writer(const void *buffer, size_t size, void *app_key) {
 	unsigned char *b, *bend;
 	(void)app_key;
 	assert(buf_offset + size < sizeof(buf));
-	memcpy(buf + buf_offset, buffer, size);
+	if (buffer) {
+		memcpy(buf + buf_offset, buffer, size);
+	}
 	b = buf + buf_offset;
 	bend = b + size;
 	printf("=> [");

tests/tests-c-compiler/check-src/check-62.c

@@ -33,7 +33,9 @@ _buf_writer(const void *buffer, size_t size, void *app_key) {
 	unsigned char *b, *bend;
 	(void)app_key;
 	assert(buf_offset + size < sizeof(buf));
-	memcpy(buf + buf_offset, buffer, size);
+	if (buffer) {
+		memcpy(buf + buf_offset, buffer, size);
+	}
 	b = buf + buf_offset;
 	bend = b + size;
 	printf("=> [");