Assorted compiler warning cleanups

Cleans up a bunch of compiler & static analysis warnings from gcc 14.1:

• IceOpenConnection: stop processing messages if connection was closed
• ConnectToPeer: return failure if malloc() fails
• EXTRACT_STRING: Avoid writing to NULL pointer if malloc fails
• _IceAddOpcodeMapping: Avoid writing to NULL pointer if malloc fails
• Clear some -Wuseless-cast warnings from gcc 14.1
• IceReadCompleteMessage: callers need to check if malloc() failed
• ProcessConnectionSetup: avoid writing to NULL pointer if malloc() failed
• ProcessProtocolSetup: return failure if malloc() failed for ProtocolName
• IceProtocolSetup: return failure if malloc() failed for authIndices
• IceRegisterForProtocolSetup: return failure if malloc() failed