indent(1): Removed whitespace shouldn't be considered in column calculations.

This piece of code removed tabs and space characters from after colons
that follow labels by decrementing the e_lab (end of label) "pointer"
which is later used to calculate the width of the string that fprintf()
puts into "output". But pad_output() gets the length from the actual
string, so it miscalculated what the current column is.

Fixed by putting a string terminator at the e_lab "pointer".

Differential Revision: https://reviews.freebsd.org/D6966
(Partial)
Obtained from: Piotr Stefaniak