diagnostics: beware of unexpected EOF when quoting the source file

When the input file contains lone CRs (aka, ^M, \r), the locations see
a new line.  Diagnostics look only at \n as end-of-line, so sometimes
there is an offset in diagnostics.  Worse yet: sometimes we loop
endlessly waiting for \n to come from a continuous stream of EOF.

Fix that:
- check for EOF
- beware not to call end_use_class if begin_use_class was not
  called (which would abort).  This could happen if the actual
  line is shorter that the expected one.

Prompted by a (private) report from Marc Schönefeld.

* src/location.c (location_caret): here.
* tests/diagnostics.at (Carriage return): New.

tests/diagnostics.at

@@ -35,17 +35,23 @@ AT_BISON_OPTION_PUSHDEFS
 
 AT_DATA_GRAMMAR([[input.y]], [$2])
 
-AT_DATA([experr.orig], [$4])
+# For some reason, literal ^M in the input are removed and don't end
+# in `input.y`.  So use the two-character ^M represent it, and let
+# Perl insert real CR characters.
+AT_CHECK([perl -pi -e 's{\^M}{\r}gx' input.y])
+
+AT_DATA([experr], [$4])
+
+AT_CHECK([LC_ALL=en_US.UTF-8 bison -fcaret --color=debug -Wall input.y], [$3], [], [experr])
 
 # When no style, same messages, but without style.
-AT_CHECK([perl -p -e 's{</?\w+>}{}g' <experr.orig >experr])
+AT_CHECK([perl -pi -e 's{(</?\w+>)}{ $[]1 eq "<tag>" ? $[]1 : "" }ge' experr])
+
 # Cannot use AT_BISON_CHECK easily as we need to change the
 # environment.
 # FIXME: Enhance AT_BISON_CHECK.
 AT_CHECK([LC_ALL=en_US.UTF-8 bison -fcaret -Wall input.y], [$3], [], [experr])
 
-AT_CHECK([cp experr.orig experr])
-AT_CHECK([LC_ALL=en_US.UTF-8 bison -fcaret --color=debug -Wall input.y], [$3], [], [experr])
 
 AT_BISON_OPTION_POPDEFS
 
@@ -255,6 +261,24 @@ input.y: <warning>warning:</warning> fix-its can be applied.  Rerun with option
 ]])
 
 
+## ----------------- ##
+## Carriage return.  ##
+## ----------------- ##
+
+# Carriage-return used to count as a newline in the scanner, and not
+# in diagnostics.  Resulting in all sort of nice bugs.
+
+AT_TEST([[Carriage return]],
+[[^M^M^M^M^M^M^M^M^M^M^M^M^M^M^M^M^M^M^M^M^M^M^M^M^M^M^M^M
+%token "
+%%
+]],
+[1],
+[[input.y:37.8-38.0: <error>error:</error> missing '"' at end of line
+input.y:37.8-38.0: <error>error:</error> syntax error, unexpected string, expecting char or identifier or <tag>
+]])
+
+
 
 m4_popdef([AT_TEST])