[reSIProcate] race condition when Data::c_str

[palladin <p-aladin@xxxxxxxxx>]

Dear resip devels,
There is small issue with rutil::Data.
Please consider next scenario:
resip::MethodNames from MethodTypes.cxx
SipMessage msg
rutil::Data d( msg.methodStr() );

When you call d.c_str() - original MethodNames will be rewritten in 
Data::c_str() when assigning zero to mBuf[mSize].

Sincerely, tez.