Use the right version of the vct_ishdrval() macro

I accidentally committed an earlier, wrong version. o/ Dridi

Ref #3407

https://github.com/varnishcache/varnish-cache/pull/3407#issuecomment-696146625

include/vct.h

@@ -84,7 +84,8 @@ vct_is(int x, uint16_t y)
 #define vct_isxmlnamestart(x) vct_is(x, VCT_XMLNAMESTART)
 #define vct_isxmlname(x) vct_is(x, VCT_XMLNAMESTART | VCT_XMLNAME)
 #define vct_istchar(x) vct_is(x, VCT_ALPHA | VCT_DIGIT | VCT_TCHAR)
-#define vct_ishdrval(x) (((x) >= 0x20 && (x) != 0x7f) || (x) == 0x09)
+#define vct_ishdrval(x) \
+    (((uint8_t)(x) >= 0x20 && (uint8_t)(x) != 0x7f) ||(uint8_t)(x) == 0x09)
 
 static inline int
 vct_iscrlf(const char* p, const char* end)