Fix name checks in ctfread.c

I noticed that ctfread.c could create a symbol with the name "".  This
happens because a couple of spots check that a name is not NULL -- but
libctf never returns such names.  Instead check the string contents.

I left the NULL checks in for robustness.

Note that other spots in ctfread.c already do check the contents of
the name.  I changed these to avoid strlen and instead check the first
character.

Approved-By: Simon Marchi <simon.marchi@efficios.com>