Bug 1427382 - Follow-up: fix compile error (signed/unsigned). rs=bustage-fix

mailnews/base/src/nsMsgDBView.cpp

@@ -425,7 +425,7 @@ nsMsgDBView::FetchAuthor(nsIMsgDBHdr * aHdr,
     if (name.IsEmpty()) {
       CopyUTF8toUTF16(emailAddress, aSenderString);
     } else {
-      uint32_t atPos;
+      int32_t atPos;
       if ((atPos = name.FindChar('@')) == kNotFound ||
           name.FindChar('.', atPos) == kNotFound) {
         aSenderString = name;