SkSL: Fail harder when trying to return a value from a void function

This prevents the inliner from getting really confused when the result
variable is null, but a return statement appears and tries to use it.

Bug: oss-fuzz:25525
Change-Id: Ief85990f707e68e104931a9778ffab35a27acfb1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/315963
Commit-Queue: Brian Osman <[email protected]>
Reviewed-by: John Stiles <[email protected]>

Author: brianosman

src/sksl/SkSLIRGenerator.cpp

@@ -747,6 +747,7 @@ std::unique_ptr<Statement> IRGenerator::convertReturn(const ASTNode& r) {
         }
         if (fCurrentFunction->fReturnType == *fContext.fVoid_Type) {
             fErrors.error(result->fOffset, "may not return a value from a void function");
+            return nullptr;
         } else {
             result = this->coerce(std::move(result), fCurrentFunction->fReturnType);
             if (!result) {