NanumGothic-ExtraBold non-0 lsbs for empty glyphs

NanumGothic-ExtraBold - and apparently none of the other styles/weights of NanumGothic - has a bunch of glyphs that are empty but have non-0 lsbs. This isn't overly harmful but the inconsistency is odd and it can confuse scripts. For example, assuming current directory is nanumgothic/:

```
import os

for file in [f for f in os.listdir('.') if f.endswith('.ttf')]:
f = ttLib.TTFont(file)
bad_lsbs = 0
for i,n in enumerate(f.getGlyphOrder()):
if f['loca'][i] != f['loca'][i+1]: continue
if f['hmtx'].metrics[n][1] != 0:
bad_lsbs += 1
print '%s has %d non-0 lsbs for empty glyphs' % (file, bad_lsbs)
```

Prints

```
NanumGothic-Bold.ttf has 0 non-0 lsbs for empty glyphs
NanumGothic-ExtraBold.ttf has 4620 non-0 lsbs for empty glyphs
NanumGothic-Regular.ttf has 0 non-0 lsbs for empty glyphs
```