Add missing EnsureSpace

My recent change
https://chromium-review.googlesource.com/c/v8/v8/+/1570666 introduced an
error, found by ClusterFuzz, because I forgot to use EnsureSpace in a
new method in assembler-x64.

Bug: chromium:959014
Change-Id: I4c1b564b05de6d4403632e1521520f87706d56c9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1594977
Auto-Submit: Seth Brenith <seth.brenith@microsoft.com>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61219}

src/x64/assembler-x64.cc

@@ -1567,6 +1567,7 @@ void Assembler::j(Condition cc,
 }
 
 void Assembler::jmp_rel(int offset) {
+  EnsureSpace ensure_space(this);
   const int short_size = sizeof(int8_t);
   const int long_size = sizeof(int32_t);
   --offset;  // This is how jumps are specified on x64.