Added better protection against internal leb128 underflow

There have already been a number of bugs that end up writing -1 out as
leb128s. The current encoder doesn't know the different betwee -1 and
0xffffffff, so asserting before this situation can happen is quite
important for preventing these bad leb128s from ever making it into a
stable version.

Also dropped LFS_ERR_OVERFLOW to use LFS_ERR_CORRUPT for bad leb128
encodings. These end up meaning the same thing to higher layers anyways.

lfs_util.c

@@ -30,7 +30,7 @@ ssize_t lfs_toleb128(uint32_t word, void *buffer, size_t size) {
         }
     }
 
-    return LFS_ERR_OVERFLOW;
+    return LFS_ERR_CORRUPT;
 }
 
 ssize_t lfs_fromleb128(uint32_t *word, const void *buffer, size_t size) {
@@ -46,7 +46,7 @@ ssize_t lfs_fromleb128(uint32_t *word, const void *buffer, size_t size) {
         }
     }
 
-    return LFS_ERR_OVERFLOW;
+    return LFS_ERR_CORRUPT;
 }