Fix for unused argument warning

The asserts are removed when compiling in non-debug mode, leading to some warnings of the form:
```
seccure/ecc.c: In function ‘point_decompress’:
seccure/ecc.c:111:12: error: variable ‘rc’ set but not used [-Werror=unused-but-set-variable]
```

The intended behavior seems to be to abort the process, so this patch does that.