This fixes nsIHttpChannel::GetRequestHeader() and
nsIHttpChannel::GetResponseHeader() to always empty out their string
argument even when they fail. This prevents programming mistakes of
passing the same string object to multiple of these calls and using the
string value without checking the nsresult error code, since otherwise
the string value may be unchanged from a previous call.
Note that this doesn't affect JS consumers of these APIs since we only
empty out the string argument in case the method fails, which will be
translated to a JS exception, and the JS code will never get to see the
emptied string.
