buffer: Don't miss return value on Windows build

... and use InterlockedExchangeAdd64 for the 64bit value.
InterlockedExchangeAdd is 32bit version.
8 jobs for missing-return in 15 minutes and 33 seconds (queued for 1 minute and 58 seconds)
Status Job ID Name Coverage
  Preparation
passed #122231
gst indent

00:00:45

passed #122230
manifest

00:00:38

 
  Build
passed #122232
build fedora x86_64

00:03:56

 
  Test
passed #122233
check fedora

00:03:36

 
  Full Builds
passed #122234
build android arm64 api28

00:03:21

passed #122236
build cerbero cross win32

00:05:55

passed #122237
build cerbero cross win64

00:05:15

passed #122235
build cerbero fedora x86_64

00:06:17