Fix fuzzing error with duplicate function definitions.

Without an early return, the inliner tries to inline a function inside
of itself, eating up gigabytes of memory before hitting its inline
threshold.

This normally wouldn't be possible because functions are meant to be
fully assembled before they're added to the list of ProgramElements, so
the inliner shouldn't find a function as a candidate to be inlined into
itself at all. However, the fuzzer found that an existing function
could be extended by re-declaring it; in this case, it is findable as
a ProgramElement and becomes inlinable.

Change-Id: I4c02a7b52e4b75151b75c94cb70dfadb8e4c9e6b
Bug: oss-fuzz:27442
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/334556
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>

src/sksl/SkSLIRGenerator.cpp

@@ -1042,6 +1042,7 @@ void IRGenerator::convertFunction(const ASTNode& f) {
                     }
                     if (other->definition() && !other->isBuiltin()) {
                         fErrors.error(f.fOffset, "duplicate definition of " + other->description());
+                        return;
                     }
                     break;
                 }