Change VTCP_Check() to take a ssize_t

Since the input value is sometimes the result of a read()/write() call,
avoid truncating the ssize_t value on calling it.

include/vtcp.h

@@ -37,7 +37,7 @@ struct suckaddr;
 #define VTCP_ADDRBUFSIZE		64
 #define VTCP_PORTBUFSIZE		16
 
-int VTCP_Check(int a);
+int VTCP_Check(ssize_t a);
 #define VTCP_Assert(a) assert(VTCP_Check(a))
 
 struct suckaddr *VTCP_my_suckaddr(int sock);

lib/libvarnish/vtcp.c

@@ -558,7 +558,7 @@ VTCP_check_hup(int sock)
  */
 
 int
-VTCP_Check(int a)
+VTCP_Check(ssize_t a)
 {
 	if (a == 0)
 		return (1);