CaptureMidExecutionSetup() currently assumes that all created programs
have also been linked, since mCachedProgramSources is only updated when
glLinkProgram() is called.   However, a call to glCreateProgram()
doesn't necessitate a call to glLinkProgram().

This leads to hitting an assert in
FrameCaptureShared::getProgramSources() when iterating all
glCreateProgram()'ed programs, but doesn't find that it in the list of
linked programs.   This fix is to move the program->isLinked() call
earlier.

Bug: angleproject:5545
Change-Id: I49fdec08de1c2767fe7f6a08fd0a9d53fa865167
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2629789Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Tim Van Patten <timvp@google.com>