BUG/MINOR: http-conv: Make url-dec failed if no space for trailing null byte

for url-dec converter, a trailing null byte is added at the end of the input
sample because it is requested by url_decode() function. However, when the
buffer was full, the last byte was crushed by the trailing null byte. In
that case, the last character was lost and not decoded.

Now, the converter just fails by returning 0.

This patch could be backported to all supported versions but it is a very
minor issue.

src/http_conv.c

@@ -273,6 +273,9 @@ static int sample_conv_url_dec(const struct arg *args, struct sample *smp, void
 		smp->flags &= ~SMP_F_CONST;
 	}
 
+	if (smp->data.u.str.size <= smp->data.u.str.data)
+		return 0;
+
 	/* Add final \0 required by url_decode(), and convert the input string. */
 	smp->data.u.str.area[smp->data.u.str.data] = '\0';