Sneaky valgrind trick to detect stack memory issues.

After something is popped from a stack, we overwrite the memory
with uninitialised data (if JQ_DEBUG is on). This means that
valgrind reports use-after-pop as an uninitialised memory error.

jv_alloc.h

@@ -3,6 +3,17 @@
 
 #include <stddef.h>
 
+#if JQ_DEBUG
+extern volatile char jv_mem_uninitialised;
+#endif
+
+static void jv_mem_invalidate(void* mem, size_t n) {
+#if JQ_DEBUG
+  char* m = mem;
+  while (n--) *m++ ^= jv_mem_uninitialised ^ jv_mem_uninitialised;
+#endif
+}
+
 void* jv_mem_alloc(size_t);
 void jv_mem_free(void*);
 __attribute__((warn_unused_result)) void* jv_mem_realloc(void*, size_t);