SHP provider: handle corrupted .SHX entries

[danstoica]

The .SHX file contains entries {geometry offset in the .SHP file, geometry size} and the entry number corresponds to the record # in the .DBF file. Found a couple of corrupted .SHX files containing "geometry size" < 0 entries and the provider didn't check for this. The result was a crash during memory allocation.

The fix consists in: a) ignore the corrupted records when building the spatial index and b) return NULL geometries to the user when hitting such records.