wininet/tests: Avoid overflowing the buffer (Coverity).

dlls/wininet/tests/http.c

@@ -613,7 +613,7 @@ static void InternetReadFile_test(int flags, const test_data_t *test)
     res = InternetQueryOptionA(hor,INTERNET_OPTION_URL,buffer,&length);
     ok(res, "InternetQueryOptionA(INTERNET_OPTION_URL) failed with error %d\n", GetLastError());
 
-    length = sizeof(buffer);
+    length = sizeof(buffer)-1;
     res = HttpQueryInfoA(hor,HTTP_QUERY_RAW_HEADERS,buffer,&length,0x0);
     ok(res, "HttpQueryInfoA(HTTP_QUERY_RAW_HEADERS) failed with error %d\n", GetLastError());
     buffer[length]=0;