Add regression test for oss-fuzz:32156.

The fuzzer found that it could overflow an int via a properly-crafted
constant-fold expression, leading to a UBSAN error. Constant-fold
expressions now guard against overflow (http://review.skia.org/413138)
and UBSAN is no longer triggered.

Change-Id: I07dba41e87bb9ceed37b84ec6b8922defbdc6550
Bug: oss-fuzz:32156, skia:12050
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/413836
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
diff --git a/gn/sksl_tests.gni b/gn/sksl_tests.gni
index ca77fd1..c46778a 100644
--- a/gn/sksl_tests.gni
+++ b/gn/sksl_tests.gni
@@ -154,6 +154,7 @@
   "/sksl/errors/Ossfuzz29849.sksl",
   "/sksl/errors/Ossfuzz31410.sksl",
   "/sksl/errors/Ossfuzz31469.sksl",
+  "/sksl/errors/Ossfuzz32156.sksl",
   "/sksl/errors/Ossfuzz32587.sksl",
   "/sksl/errors/Ossfuzz32851.sksl",
   "/sksl/errors/OverflowFloatLiteral.sksl",