Fix off-by-one error in concat with fixlastline="yes"

PR: 25464
Obtained from: wang liang


git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@275773 13f79535-47bb-0310-9956-ffa450edef68

src/main/org/apache/tools/ant/taskdefs/Concat.java

@@ -797,7 +797,6 @@ public class Concat extends Task {
                     }
                     continue;
                 }
-
                 int nRead = getReader().read(cbuf, off, len);
                 if (nRead == -1 || nRead == 0) {
                     reader.close();
@@ -816,7 +815,7 @@ public class Concat extends Task {
                             if (i < 0) {
                                 break;
                             }
-                            addLastChar(cbuf[off + i]);
+                            addLastChar(cbuf[off + i - 1]);
                         }
                     }
                     len -= nRead;