Fix compile time error in SparseData.h (#566)

In some platforms, it would error out like:

In file included from SparseData.c:21:
In function ‘compword_to_int8’,
    inlined from ‘sdata_to_float8arr’ at SparseData.c:248:14:
SparseData.h:291:16: error: ‘num’ may be used uninitialized [-Werror=maybe-uninitialized]
  291 |         return num;
      |                ^~~
SparseData.c: In function ‘sdata_to_float8arr’:
SparseData.h:255:15: note: ‘num’ declared here
  255 |         int64 num;
      |               ^~~
In function ‘compword_to_int8’,
    inlined from ‘sdata_index_to_int64arr’ at SparseData.c:274:17:
SparseData.h:291:16: error: ‘num’ may be used uninitialized [-Werror=maybe-uninitialized]
  291 |         return num;
      |                ^~~
SparseData.c: In function ‘sdata_index_to_int64arr’:
SparseData.h:255:15: note: ‘num’ declared here
  255 |         int64 num;
      |               ^~~
cc1: all warnings being treated as errors

Author: reshke

gpcontrib/gp_sparse_vector/SparseData.h

@@ -284,6 +284,8 @@ static inline int64 compword_to_int8(const char *entry)
 			numptr8[6] = entry[7];
 			numptr8[7] = entry[8];
 			break;
+		default:
+			abort();
 	}
 
 	return num;