Update overview for the zero-byte OOB case (WebAssembly#100)

It's only a trap if a byte is accessed out-of-bounds, not if the
destination or source index is out-of-bounds. This only makes a
difference when the length is zero, in which case the previous behavior
would trap, and the new behavior will be a no-op.

Author: binji

proposals/bulk-memory-operations/Overview.md

@@ -277,8 +277,6 @@ A trap occurs if:
   active segments that were dropped after being copied into memory during module
   instantiation.
 * any of the accessed bytes lies outside the source data segment or the target memory
-* the source offset is greater than the length of the source data segment
-* the destination offset is greater than the length of the target memory
 
 Note that it is allowed to use `memory.init` on the same data segment more than
 once.
@@ -342,8 +340,6 @@ The instruction has the signature `[i32 i32 i32] -> []`. The parameters are, in
 
 A trap occurs if:
 * any of the accessed bytes lies outside the source or target memory
-* the source offset is greater than the length of the source memory
-* the destination offset is greater than the length of the target memory
 
 A trap resulting from an access outside the source or target region
 only occurs once the first byte that is outside the source or target
@@ -367,7 +363,6 @@ The instruction has the signature `[i32 i32 i32] -> []`. The parameters are, in
 
 A trap occurs if:
 * any of the accessed bytes lies outside the target memory
-* the destination offset is greater than the length of the target memory
 
 Filling takes place bytewise from lower addresses toward higher
 addresses.  A trap resulting from an access outside the target memory