The new font loading functions introduced in
544e62ac assume that popen() returns
NULL when fc-match is unavailable or fails. This is incorrect, since
popen() will always start a shell to run the passed command and return a
stream to that shell's stdin and stdout.

This results in an non-null but empty font name being passed to
FT_New_Face(), which fails.

This commit fixes this by also using the fallback font when the font
path read from the popen() stream is empty.

Fixes #239
Fixes regression caused by 544e62ac