Curl_rtsp_parseheader: avoid useless malloc/free

Coverity actually pointed out flawed logic in the previous call to
Curl_strntoupper() where the code used sizeof() of a pointer to pass in
a size argument. That code still worked since it only needed to
uppercase 4 letters. Still, the entire malloc/uppercase/free sequence
was pointless since the code has already matched the string once in the
condition that starts the block of code.