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

jv_alloc.c

@@ -169,8 +169,11 @@ void* jv_mem_realloc(void* p, size_t sz) {
 #ifndef NDEBUG
 volatile char jv_mem_uninitialised;
 __attribute__((constructor)) void jv_mem_uninit_setup(){
+  // ignore warning that this reads uninitialized memory - that's the point!
+#ifndef __clang_analyzer__
   char* p = malloc(1);
   jv_mem_uninitialised = *p;
   free(p);
+#endif
 }
 #endif