fc: Handle fonts with broken metrics better

It seems that some fonts don't have meaningful
values for underline and strikethrough metrics.

Ignore these font-provided values if they are zero.

Fixes: #616
5 jobs for broken-underlines in 7 minutes and 41 seconds (queued for 6 seconds)
Status Name Job ID Coverage
  Build
passed linux-fedora #1663195

00:00:55

passed macos #1663197
macos

00:07:39

passed msys2-mingw64 #1663196
win32-ps

00:03:31

 
  Analysis
passed asan-build #1663198
asan

00:00:45

 
  Docs
passed reference #1663199

00:01:02