install-ffmpeg-android.sh: Assert OpenSSL symlinks

In the script, we currently remove symlinks if they are not already
there, then establish and then remove them again. This is not necessary,
the symlinks should already be in the prebuilt binaries. This patch
makes us assert that they are there.

As a drive-by, we move some logic around to confirm the OpenSSL include
folder is valid as early as possible.

Pick-to: 6.12
Change-Id: Id7a74913be8b3c3c27350d4eee8259312c25f740
Reviewed-by: Artem Dyomin <artem.dyomin@qt.io>