Fix for sendfile(), when the protocol char (mnemonic) is the space (ASCII 32)

char, don't try to match it with a transfer protocol. When a user doesn't have
a default transfer protocol selected, their "prot" property may be a space.

src/sbbs3/download.cpp

@@ -457,7 +457,7 @@ bool sbbs_t::sendfile(char* fname, char prot)
 	size_t	i;
 	bool	result=false;
 
-	if(prot)
+	if(prot > ' ')
 		ch=toupper(prot);
 	else {
 		xfer_prot_menu(XFER_DOWNLOAD);