Don't blow up on read frontier violations.

This is a useful debugging message and we'd like to know when it
happens, but it's more accurately backstopped by the target window
checksum. We should rely on that instead.

Change-Id: I7658a874a8ab1cf1e047984d5829657041336968
diff --git a/syspatch.c b/syspatch.c
index 0b7f50b..a90e031 100644
--- a/syspatch.c
+++ b/syspatch.c
@@ -123,10 +123,8 @@
     MapState* state = (MapState*) source->ioh;
     source_read->blkno = source->getblkno;
     source_read->length = read_with_map(source_read->data, sizeof(source_read->data), state);
-    if (ftell(state->f) < READ_FRONTIER) {
+    if (ftell(state->f) < READ_FRONTIER)
         printf("read past frontier: %ld < %zu\n", ftell(state->f), READ_FRONTIER);
-        return NULL;
-    }
 
     return source_read;
 }