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.h

@@ -88,8 +88,6 @@ enum lfs_error {
     LFS_ERR_NOATTR      = -61,  // No data/attr available
     LFS_ERR_NAMETOOLONG = -36,  // File name too long
     LFS_ERR_RANGE       = -34,  // Result out of range
-    // TODO should all overflow errors actually be corrupt errors?
-    LFS_ERR_OVERFLOW    = -75,  // Value too large for defined data type
 };
 
 // File types