diff_print: return out-of-memory situation when printing binary

We currently don't check for out-of-memory situations on exiting
`format_binary` and, as a result, may return a partially filled buffer.
Fix this by checking the buffer via `git_buf_oom`.

src/diff_print.c

@@ -491,6 +491,9 @@ static int format_binary(
 	}
 	git_buf_putc(pi->buf, '\n');
 
+	if (git_buf_oom(pi->buf))
+		return -1;
+
 	return 0;
 }