8261949: fileStream::readln returns incorrect line string

Reviewed-by: dcubed, dholmes

Author: y1yang0

src/hotspot/share/utilities/ostream.cpp

@@ -589,8 +589,11 @@ char* fileStream::readln(char *data, int count ) {
  char * ret = NULL;
  if (_file != NULL) {
  ret = ::fgets(data, count, _file);
- //Get rid of annoying \n char
- data[::strlen(data)-1] = '\0';
+ // Get rid of annoying \n char only if it is present.
+ size_t len = ::strlen(data);
+ if (len > 0 && data[len - 1] == '\n') {
+ data[len - 1] = '\0';
+ }
  }
  return ret;
 }