SMTP: escape ASCII strings printed to COL_INFO

This is already done automatically for the hf entries thanks to hfinfo_format_text() function

Bug: 10536
Change-Id: I9ee56b795234a94f0e59c82a96e3e6cedf71c4e6
Reviewed-on: https://code.wireshark.org/review/5459
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
(cherry picked from commit 78849d0e6b0dde760c8bea40ce99e7a05711f3bf)
Conflicts:
	epan/dissectors/packet-smtp.c
Reviewed-on: https://code.wireshark.org/review/5470