Store the PEM certificate in a dynamically allocated buffer

Store the PEM certificate in a dynamically allocated buffer

... when https-inspecting.

Should prevent errors like:
2021-03-16 22:36:19.148 7f47bbfff700 Error: X509 PEM cert len 16694 is larger than buffer len 16383

As a bonus it should slightly reduce the memory usage as most
certificates are smaller than the previously used fixed buffer.

Reported by: Wen Yue