Fixed several issues with compat flag parsing

- Fixed issue where some overflowed compat flags could end up ignored.

  A simple typo: incrementing by the unrelated d variable, meant we
  were skipping overflowed compat flags whenever the previous logic sets
  d > 1.

- Fixed issue where any zero padding was treated as overflowed compat
  flags.

  Note this hid the previous issue from our tests.

Added more tests to prevent a regression here. Letting bad compat flag
parsing through would be _very_ annoying in the future.

Code changes:

           code          stack          ctx
  before: 38148           2608          752
  after:  38084 (-0.2%)   2608 (+0.0%)  752 (+0.0%)

lfs.c

@@ -13471,16 +13471,16 @@ static int lfsr_data_readrcompat(lfs_t *lfs, lfsr_data_t *data,
     //
     // we don't really care about performance here
     while (lfsr_data_size(*data) > 0) {
-        lfs_scmp_t cmp = lfsr_data_cmp(lfs, *data, (uint8_t[]){0}, 1);
-        if (cmp < 0) {
-            return cmp;
+        uint8_t b;
+        lfs_ssize_t d = lfsr_data_read(lfs, data, &b, 1);
+        if (d < 0) {
+            return d;
         }
 
-        if (cmp != LFS_CMP_EQ) {
+        if (b != 0x00) {
             *rcompat |= LFSR_RCOMPAT_OVERFLOW;
+            break;
         }
-
-        *data = LFSR_DATA_SLICE(*data, d, -1);
     }
 
     return 0;