mwindow: make sure the whole range is contained inside the same window

Looking through the open windows to check whether we can re-use an
open window should take into account whether both `offset` and `offset
+ extra` are contained within the same window. Failure to do so can
lead to invalid memory accesses. This closes #614.

While we're in the area remove an outdated assert.

src/mwindow.c

@@ -211,13 +211,15 @@ unsigned char *git_mwindow_open(
 	git_mwindow_ctl *ctl = &GIT_GLOBAL->mem_ctl;
 	git_mwindow *w = *cursor;
 
-	if (!w || !git_mwindow_contains(w, offset + extra)) {
+	if (!w || !(git_mwindow_contains(w, offset) &&
+		    git_mwindow_contains(w, offset + extra))) {
 		if (w) {
 			w->inuse_cnt--;
 		}
 
 		for (w = mwf->windows; w; w = w->next) {
-			if (git_mwindow_contains(w, offset + extra))
+			if (git_mwindow_contains(w, offset) &&
+			    git_mwindow_contains(w, offset + extra))
 				break;
 		}
 
@@ -242,7 +244,6 @@ unsigned char *git_mwindow_open(
 	}
 
 	offset -= w->offset;
-	assert(git__is_sizet(offset));
 
 	if (left)
 		*left = (unsigned int)(w->window_map.len - offset);