ast: Fix nullptr deref in Variable::info_to_str

Varaibles can infer types now, in which case the type_ field is null.

Fixed: chromium:1221120
Change-Id: I0cb2a6a2e8128c56625f48940cf73cf4cadb22ce
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/55252
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/ast/variable.cc

@@ -85,7 +85,10 @@ void Variable::info_to_str(const sem::Info& sem,
   make_indent(out, indent);
   out << declared_access_ << std::endl;
   make_indent(out, indent);
-  out << type_->type_name() << std::endl;
+  if (type_) {
+    out << type_->type_name();
+  }
+  out << std::endl;
 }
 
 void Variable::constructor_to_str(const sem::Info& sem,

test/bug/chromium/1221120.wgsl

@@ -0,0 +1,2 @@
+let
+H=1;

test/bug/chromium/1221120.wgsl.expected.hlsl

@@ -0,0 +1,6 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+static const int H = 1;

test/bug/chromium/1221120.wgsl.expected.msl

@@ -0,0 +1,4 @@
+#include <metal_stdlib>
+
+using namespace metal;
+constant int H = 1;

test/bug/chromium/1221120.wgsl.expected.spvasm

@@ -0,0 +1,19 @@
+; SPIR-V
+; Version: 1.3
+; Generator: Google Tint Compiler; 0
+; Bound: 7
+; Schema: 0
+               OpCapability Shader
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint GLCompute %unused_entry_point "unused_entry_point"
+               OpExecutionMode %unused_entry_point LocalSize 1 1 1
+               OpName %H "H"
+               OpName %unused_entry_point "unused_entry_point"
+        %int = OpTypeInt 32 1
+          %H = OpConstant %int 1
+       %void = OpTypeVoid
+          %3 = OpTypeFunction %void
+%unused_entry_point = OpFunction %void None %3
+          %6 = OpLabel
+               OpReturn
+               OpFunctionEnd

test/bug/chromium/1221120.wgsl.expected.wgsl

@@ -0,0 +1 @@
+let H = 1;