Allocate SplashXPathScanner on the stack

SplashXPathScanner is only used inside the each fill function, but newer
passed to the outside. As it is small, there is no reason to not
allocate it on the stack.
7 jobs for reduce_heap_allocations in 13 minutes and 50 seconds (queued for 11 seconds)
latest
Status Job ID Name Coverage
  Build
passed #71313
build

00:04:11

passed #71318
build_android

00:02:11

passed #71314
build_clang_libcpp

00:05:21

passed #71317
build_clazy

00:04:11

passed #71315
build_gcc_49

00:04:10

passed #71319
build_mingw64_fedora

00:06:36

failed #71316
build_mingw64_fedora

00:00:15