Refind import cuesheet op after appending seekpoints (#677)

This fixes a heap-use-after-free. The free was part of a realloc,
and the cuesheet op handle was still pointing to the old
allocation

Credit: Oss-Fuzz
Issue: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=61292

src/metaflac/options.c

@@ -251,8 +251,11 @@ FLAC__bool parse_options(int argc, char *argv[], CommandLineOptions *options)
 		Operation *op = find_shorthand_operation(options, OP__IMPORT_CUESHEET_FROM);
 		if(0 != op) {
 			Operation *op2 = find_shorthand_operation(options, OP__ADD_SEEKPOINT);
-			if(0 == op2)
+			if(0 == op2) {
 				op2 = append_shorthand_operation(options, OP__ADD_SEEKPOINT);
+				/* Need to re-find op, because the appending might have caused realloc */
+				op = find_shorthand_operation(options, OP__IMPORT_CUESHEET_FROM);
+			}
 			op->argument.import_cuesheet_from.add_seekpoint_link = &(op2->argument.add_seekpoint);
 		}
 	}