gwinhttpvfs: Fall back to wrapped VFS if creating a HTTP file fails

If we fail to create a GWinhttpFile for a URI (for example, because it’s
an invalid URI or is badly encoded), don’t just return NULL. Instead,
fall back to the wrapped VFS which might be able to handle it instead.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Fixes: #1819
10 jobs for 1819-file-for-uri-null in 33 minutes and 15 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #352325
cross-android_api21_arm64

00:05:35

passed #352326
cross-android_api28_arm64

00:05:35

passed #352327
cross-mingw64

00:05:24

passed #352323
debian-stable-x86_64

00:08:48

passed #352322
fedora-x86_64

00:13:05

passed #352324
G_DISABLE_ASSERT

00:09:06

passed #352328
win32
msys2-mingw32

00:14:40

passed #352329
win32
vs2017-x64

00:09:47

 
  Coverage
passed #352330
coverage

00:04:35

77.8%
 
  Analysis
passed #352331
scan-build

00:13:59