Fix crash when replying to a mail with umlauts in the subject.

The cause for this crash was an out-of-bounds array access.
The same problem is in KMail in 3.5 branch, but it doesn't crash. I think Qt4 introduced a new assert when accessing a QString.

svn path=/trunk/KDE/kdepim/; revision=667702

kmmsgbase.cpp

@@ -569,7 +569,7 @@ QByteArray KMMsgBase::encodeRFC2047String(const QString& _str,
         }
         stop = start + dif;
       }
-      p = stop;
+      p = (stop >= strLength ? strLength - 1 : stop);
       while (p > start && _str.at(p) != ' ') p--;
       if (p > start) stop = p;
       if (result.right(3) == "?= ") start--;