Fix assertion when returning intermediate types.

The fuzzer managed to trigger an assertion by returning an invalid type
from a void function. We were neglecting to clear out the expression
when reporting it as invalid, leaving it for `checkValid` to find later.

Change-Id: Icc152c867a3316fe994967e192601fb4d10da98f
Bug: oss-fuzz:37704
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/442678
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>

resources/sksl/errors/ReturnFromVoid.sksl

@@ -1 +1,3 @@
-void main() { return true; }
+void a() { return true; }
+void b() { return b; }
+void c() { return int; }

src/sksl/SkSLIRGenerator.cpp

@@ -801,6 +801,7 @@ std::unique_ptr<Block> IRGenerator::finalizeFunction(const FunctionDeclaration&
                                     std::move(returnStmt.expression()), returnType));
                         } else {
                             // Returning something from a function with a void return type.
+                            returnStmt.setExpression(nullptr);
                             fIRGenerator->errorReporter().error(returnStmt.fOffset,
                                                      "may not return a value from a void function");
                         }

tests/sksl/errors/ReturnFromVoid.glsl

@@ -1,4 +1,6 @@
 ### Compilation failed:
 
 error: 1: may not return a value from a void function
-1 error
+error: 2: may not return a value from a void function
+error: 3: may not return a value from a void function
+3 errors