fix errors flagged by clang static analyzer

builtin.c: bug - free of uninitialized jv
compile.c: missing assertion
jq_test.c: buggy logic / unreachable code
jv.c: missing assertion
jv_alloc.c: false positive - intentional read of uninitialized memory
jv_file.c: dead code

compile.c

@@ -943,7 +943,7 @@ block gen_cbinding(const struct cfunction* cfunctions, int ncfunctions, block co
 
 static uint16_t nesting_level(struct bytecode* bc, inst* target) {
   uint16_t level = 0;
-  assert(bc && target->compiled);
+  assert(bc && target && target->compiled);
   while (bc && target->compiled != bc) {
     level++;
     bc = bc->parent;