fix truncateForError() handling of carriage return

[WalterBright]

The original skips the character after \r is removed.

[github-actions]

DMD perf check

Metric	Base	PR	Δ
dmd binary size (stripped)	6.86 MB	6.86 MB	-0.11%

All measurements

Metric	Base	PR	Δ
compile hello.d (instr)	214.9 M	214.9 M	+0.010%
compile hello.d -O -release (instr)	233.2 M	233.2 M	+0.010%
compile Phobos (instr)	5,123.7 M	5,123.7 M	-0.001%
compile Phobos codegen (instr)	1,472.7 M	1,472.5 M	-0.017%
compile vibe.d (instr)	15,130.9 M	15,131.2 M	+0.002%
dmd binary size (stripped)	6.86 MB	6.86 MB	-0.11%
hello binary size (stripped)	0.72 MB	0.72 MB	0.00%
peak RSS (compile hello.d)	43.42 MB	43.40 MB	-0.05%
peak RSS (compile Phobos)	620.1 MB	618.7 MB	-0.23%
peak RSS (compile vibe.d)	1917 MB	1918 MB	+0.03%
compile dmd itself (wall)	12.2 s	12.1 s	-0.46%
compile hello.d (wall)	67.2 ms	66.3 ms	-1.36%
compile Phobos (wall)	1,522 ms	1,530 ms	+0.55%

_{211db4f vs merge-base b41cb91 · about these metrics}