r336028 changed next_msg to a char * from char [] of fixed size. Change
2nd argument of vsnprintf() to get the strlen of next_msg so that the
appropriate size is used.

Found with gcc.

/usr.bin/top/display.c: In function 'new_message':
/usr.bin/top/display.c:963:31: error:
argument to 'sizeof' in 'vsnprintf' call is the same expression as the
destination; did you mean to provide an explicit length?
[-Werror=sizeof-pointer-memaccess]

vsnprintf(next_msg, sizeof(next_msg), msgfmt, args);

Reviewed by: daichi