TSHttpTxnErrorBodySet() can leak memory (pt 2)

TS-826 resolved a memory leak with TSHttpTxnErrorBodySet but it appears that mimetype is still being leaked. See HttpSM::setup_internal_transfer line 5416 which frees internal_msg_buffer_type...it's expected that mimetype was malloced since clearly it's being freed. So that means there is still a memory leak in TSHttpTxnErrorBodySet().

Patch included.