Fix iteration issue when chains have zero-length

Fixes #27. The issue here is that `iterate` for `ChainedVector` was
assuming that underlying array chains wouldn't have zero-length. If a
user is doing a lot of filtering/deleting, however, it might be the case
that certain chunks end up with zero-length (though we do try to prune
those out when possible, so I'm still a little unsure how we get in this
state). Nevertheless, this assumption is a bit optimistic, and we can do
better by just checking if the next chunk is zero-length when iterating
and moving on to the next chunk if so.

src/chainedvector.jl

@@ -50,28 +50,43 @@ end
 # efficient iteration
 @inline function Base.iterate(A::ChainedVector)
     length(A) == 0 && return nothing
-    i = 2
+    i = 1
     chunk = 1
     chunk_i = 1
     chunk_len = A.inds[1]
-    if i > chunk_len
+    while i > chunk_len
         chunk += 1
-        chunk_i = 1
-        @inbounds chunk_len = A.inds[min(length(A.inds), chunk)]
+        @inbounds chunk_len = A.inds[chunk]
+        i <= chunk_len && break
+    end
+    x = A.arrays[chunk][1]
+    # find next valid index
+    i += 1
+    if i > chunk_len
+        while true
+            chunk += 1
+            chunk > length(A.inds) && break
+            @inbounds chunk_len = A.inds[chunk]
+            i <= chunk_len && break
+        end
     else
         chunk_i += 1
     end
-    return A.arrays[1][1], (i, chunk, chunk_i, chunk_len, length(A))
+    return x, (i, chunk, chunk_i, chunk_len, length(A))
 end
 
 @inline function Base.iterate(A::ChainedVector, (i, chunk, chunk_i, chunk_len, len))
     i > len && return nothing
     @inbounds x = A.arrays[chunk][chunk_i]
     i += 1
     if i > chunk_len
-        chunk += 1
         chunk_i = 1
-        @inbounds chunk_len = A.inds[min(length(A.inds), chunk)]
+        while true
+            chunk += 1
+            chunk > length(A.inds) && break
+            @inbounds chunk_len = A.inds[chunk]
+            i <= chunk_len && break
+        end
     else
         chunk_i += 1
     end