Fix Nasal GC errors on tests/reset

Ensure the Context temps are cleared, and when recycling an naCode,
ensure old values are cleared explicily.

Sentry-Id: FLIGHTGEAR-Y

simgear/nasal/code.c

@@ -248,7 +248,7 @@ void naFreeContext(naContext c)
     // than I have right now. So instead I'm clearing the stack tops here, so
     // a freed context looks the same as a new one returned by initContext.
 
-    c->fTop = c->opTop = c->markTop = 0;
+    c->fTop = c->opTop = c->markTop = c->ntemps = 0;
 
     c->nextFree = globals->freeContexts;
     globals->freeContexts = c;

simgear/nasal/misc.c

@@ -100,7 +100,14 @@ naRef naNewHash(struct Context* c)
 
 naRef naNewCode(struct Context* c)
 {
-    return naNew(c, T_CODE);
+    naRef r = naNew(c, T_CODE);
+    // naNew can return a previously used naCode. naCodeGen will init
+    // all these members but a GC can occur inside naCodeGen, so we see
+    // partially initalized state here. To avoid this, clear out the values
+    // which mark() cares about.
+    PTR(r).code->srcFile = naNil();
+    PTR(r).code->nConstants = 0;
+    return r;
 }
 
 naRef naNewCCode(struct Context* c, naCFunction fptr)