llvm-cov: When reading strings in gcov data, skip leading zeros

It seems that gcov, when faced with a string that is apparently zero
length, just keeps reading words until it finds a length it likes
better. I'm not really sure why this is, but it's simple enough to
make llvm-cov follow suit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204881 91177308-0d34-0410-b5e6-96231b3b80d8