Before 3793caa5 the code was
"if (...) do { ... } while (...);". After said commit this became
"if (...) av_assert0(...); do { ... } while (...);", i.e. the loop
is always executed. This commit changes the logic to what it was before
said commit. Notice that the condition is always true in FATE, so no
changes are necessary there.

This fixes a -Wmisleading-indentation warning from GCC 11.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>