Return nil on SkSL to program failure in Metal.

Bug: chromium:985129
Change-Id: I7dfea73f7eb48aeba11c110fc8b9fa253770afc1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/229493
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
Auto-Submit: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>

src/gpu/mtl/GrMtlUtil.mm

@@ -158,6 +158,7 @@ id<MTLLibrary> GrCompileMtlShaderLibrary(const GrMtlGpu* gpu,
     if (!program) {
         SkDebugf("SkSL error:\n%s\n", gpu->shaderCompiler()->errorText().c_str());
         SkASSERT(false);
+        return nil;
     }
 
     *outInputs = program->fInputs;