It is already cut off in the stat file:
19211 (nac-logging-amq) S 1 19211 19211 0 -1 4202752 5873068 0 48 0 2444657 118010 0 0 20 0 1 0 363988890 143618048 3147 18446744073709551615 4194304 5777388 140726113689888 140726113689176 140479796634259 0 0 4224 0 0 0 0 17 0 0 0 16 0 0 7874544 7887424 30507008 140726113697094 140726113697600 140726113697600 140726113697736 0
It seems this is a hard limit of the proc filesystem: linux - What is the maximum allowed limit on the length of a process name? - Stack Overflow
The cmdline file contains the full name which could be used to extract the process name instead of the stat file.