fixup for sip_utils

fix for patch:
0f524801 PJSIP support to std::error_code and std::system_error

pj_strerror crashes on due to wrong given string size.

Change-Id: I8d68b885ce0d587b27e8b361c9ad60019ed355fb

src/sip/sip_utils.cpp

@@ -49,7 +49,7 @@ PjsipErrorCategory::message( int condition ) const
 {
     std::string err_msg;
     err_msg.reserve(PJ_ERR_MSG_SIZE);
-    err_msg.resize(pj_strerror(condition, &err_msg[0], err_msg.size()).slen);
+    err_msg.resize(pj_strerror(condition, &err_msg[0], err_msg.capacity()).slen);
     return err_msg;
 }