fix bugs in new stretchy buffer code, add stretchy buffer tests

tests/stretch_test.c

@@ -0,0 +1,24 @@
+#include "stretchy_buffer.h"
+#include <assert.h>
+
+int main(int arg, char **argv)
+{
+   int i;
+   int *arr = NULL;
+
+   for (i=0; i < 1000000; ++i)
+      sb_push(arr, i);
+
+   assert(sb_count(arr) == 1000000);
+   for (i=0; i < 1000000; ++i)
+      assert(arr[i] == i);
+
+   sb_free(arr);
+   arr = NULL;
+
+   for (i=0; i < 1000; ++i)
+      sb_add(arr, 1000);
+   assert(sb_count(arr) == 1000000);
+
+   return 0;
+}