libdrgn: dwarf_index: fix memcpy() undefined behavior

Apparently, it's undefined behavior to pass NULL as the source to
memcpy(), even if the length is zero. It's an easy fix, so let's appease
UBSan.

libdrgn/dwarf_index.c

@@ -602,15 +602,18 @@ static struct drgn_error *read_cus(struct drgn_dwarf_index *dindex,
 			}
 		}
 
-		#pragma omp critical(drgn_read_cus)
-		if (!err) {
-			if (compilation_unit_vector_reserve(all_cus,
-							    all_cus->size + cus.size)) {
-				memcpy(all_cus->data + all_cus->size, cus.data,
-				       cus.size * sizeof(*cus.data));
-				all_cus->size += cus.size;
-			} else {
-				err = &drgn_enomem;
+		if (cus.size) {
+			#pragma omp critical(drgn_read_cus)
+			if (!err) {
+				if (compilation_unit_vector_reserve(all_cus,
+								    all_cus->size + cus.size)) {
+					memcpy(all_cus->data + all_cus->size,
+					       cus.data,
+					       cus.size * sizeof(*cus.data));
+					all_cus->size += cus.size;
+				} else {
+					err = &drgn_enomem;
+				}
 			}
 		}