av_guess_sample_aspect_ratio() will return undefined or missing
value as {0,1}. This fixes show_stream() to check numerator to
display 'N/A' when appropriate. show_frame() does this already
correctly.
Signed-off-by: Timo Teräs <timo.teras@iki.fi>
(cherry picked from commit c663dce031b3973e37c83ae1818f1484e1cf482c)