fix line counting with mixed CR and LF

A CR by itself could be incorrectly treated as preceding a later LF,
which would form a CRLF combination if they had been adjacent.

Closes #3033

pkgs/racket-test-core/tests/racket/port.rktl

@@ -683,6 +683,21 @@
       (test 0 syntax-column stx)
       (test 1026 syntax-position stx))))
 
+;; Test provided by @wcs4217
+(let ([p (open-input-bytes #"\rx\ny")])
+  (port-count-lines! p)
+  (test-values '(1 0 1) (lambda () (port-next-location p)))
+  (test #\return read-char p)
+  (test-values '(2 0 2) (lambda () (port-next-location p)))
+  (test #\x read-char p)
+  (test-values '(2 1 3) (lambda () (port-next-location p)))
+  (test #\newline read-char p)
+  (test-values '(3 0 4) (lambda () (port-next-location p)))
+  (test #\y read-char p)
+  (test-values '(3 1 5) (lambda () (port-next-location p)))
+  (test eof read-char p)
+  (test-values '(3 1 5) (lambda () (port-next-location p))))
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;  Check that if the initial commit thread is killed, then
 ;;  another commit thread is broken, that the second doesn't

racket/src/racket/src/port.c

@@ -1043,6 +1043,7 @@ XFORM_NONGCING static void do_count_lines(Scheme_Port *ip, const char *buffer, i
       ip->column = 0;
   } else {
     ip->charsSinceNewline += c;
+    ip->was_cr = 0;
   }
 
   /* Do the last line to get the column count right and to