otp-test-wrapper.sh: Prevent division-by-zero error when file has no executable lines.

If a source file has zero lines of executable code (one that only defines a behaviour, for example), otp-test-wrapper.sh dies with a division-by-zero error, preventing the package from being built.

I tried to build a test package with my changes, but "make package" failed:
```
Making check in tests
FAIL: test-template
FAIL: test-hooks
PASS: test-oldrecord.sh
```