[xst fuzzer] Crashing test case in `fxCheckNativeConstructor`

[gal1ium]

Build environment: Linux
Moddable SDK version: 6.0.0 55293e1

Description
Hi, we found a crashing test case when running the xst fuzzing driver (https://github.com/Moddable-OpenSource/moddable/blob/public/xs/tools/xstFuzz.c).

Steps to Reproduce

1. Build the project based on https://github.com/google/oss-fuzz/tree/master/projects/xs
2. Run xst ./poc

PoC

class extends class}

Output:
Segmentation fault (core dumped)

Crashing thread backtrace:

#0  0x000000000056b2d0 in fxCheckNativeConstructor 
                       at /src/moddable/xs/sources/xsSyntaxical.c:3947
#1  0x000000000056b2d0 in fxClassExpression 
                       at /src/moddable/xs/sources/xsSyntaxical.c:2612
#2  0x000000000056ea1a in fxLiteralExpression 
                       at /src/moddable/xs/sources/xsSyntaxical.c:2440
#3  0x000000000056d075 in fxCallExpression 
                       at /src/moddable/xs/sources/xsSyntaxical.c:2124
#4  0x0000000000569a7e in fxClassExpression 
                       at /src/moddable/xs/sources/xsSyntaxical.c:2610
#5  0x0000000000565918 in fxStatement 
                       at /src/moddable/xs/sources/xsSyntaxical.c:1270
#6  0x00000000005686d4 in fxProgram 
                       at /src/moddable/xs/sources/xsSyntaxical.c:1155
#7  0x00000000005774ef in fxParserTree 
                       at /src/moddable/xs/sources/xsTree.c:222
#8  0x000000000052fd42 in fxParseScript 
                       at /src/moddable/xs/sources/xsPlatforms.c:332
#9  0x00000000005c0b54 in fuzz_oss 
                       at /src/moddable/xs/tools/xstFuzz.c:670
#10 0x00000000005bffcd in LLVMFuzzerTestOneInput 
                       at /src/moddable/xs/tools/xstFuzz.c:82

[phoddie]

Thanks! We turned this one up a few days ago. We're working on a fix. There are other paths to get there:

class    _  extends  class A  extends  class   { 1  }