Simplify WS allocation in tlv_string

Patch by @ehocdet, commit message edited by @nigoroll:

The root cause of #3131 was misdiagnosed to the extent that, while this
change had prevented it, the root cause was a bug in WS_ReserveSize()
fixed in 505b7bd9

The previous tlv_string() code was correct except for the
fact that error handling should have checked for WS_ReserveSize(ctx->ws,
len+1) <= len (also spotted by @ehocdet).

Someone had mentioned at some point that we would not want to VRT_fail(),
but I think this must have been related to the proxy transport code, not
the proxy vmod.

Ref varnishcache/varnish-cache#3131

(cherry picked from commit e74f9e87)

lib/libvmod_proxy/vmod_proxy.c

@@ -105,12 +105,13 @@ tlv_string(VRT_CTX, int tlv)
 
 	if (VPX_tlv(ctx->req, tlv, (void **)&dst, &len))
 		return (NULL);
-	if (!WS_ReserveSize(ctx->ws, len+1))
+	d = WS_Alloc(ctx->ws, len+1);
+	if (d == NULL) {
+		VRT_fail(ctx, "proxy.TLV: out of workspace");
 		return (NULL);
-	d = ctx->ws->f;
+	}
 	memcpy(d, dst, len);
 	d[len] = '\0';
-	WS_Release(ctx->ws, len+1);
 	return (d);
 }