Fix iconv encoding of long strings

* libguile/print.c (display_string_using_iconv): If the encoding the
  full utf8 buffer would overflow the output buffer, just keep trucking
  instead of erroring.  Fixes #22667.
* test-suite/tests/iconv.test ("round-trip"): Add some tests.

libguile/print.c

@@ -1034,7 +1034,11 @@ display_string_using_iconv (const void *str, int narrow_p, size_t len,
 
 	      printed++;
 	    }
-	  else
+	  else if (errno_save == E2BIG)
+            /* No space in output buffer for this input.  Keep
+               trucking.  */
+            continue;
+          else
 	    /* Something bad happened that we can't handle: bail out.  */
 	    break;
 	}

test-suite/tests/iconv.test

@@ -118,3 +118,16 @@
       (equal? (make-string (string-length s) #\?)
               (bytevector->string (string->bytevector s "ascii" 'substitute)
                                   "ascii")))))
+
+(with-test-prefix "round-trip"
+  (define (pass-if-round-trip str encoding)
+    (pass-if-equal str str
+      (bytevector->string (string->bytevector str encoding) encoding)))
+
+  (pass-if-round-trip (make-string 128 #\a) "UTF-8")
+  (pass-if-round-trip (make-string 128 #\a) "UTF-16")
+  (pass-if-round-trip (make-string 128 #\a) "UTF-32")
+
+  (pass-if-round-trip (make-string 129 #\a) "UTF-8")
+  (pass-if-round-trip (make-string 129 #\a) "UTF-16")
+  (pass-if-round-trip (make-string 129 #\a) "UTF-32"))