Add sdsnative()

Use the existing memory space for an SDS to convert it to a regular
character buffer so we don't need to allocate duplicate space just
to extract a usable buffer for native operations.

src/sds.c

@@ -88,6 +88,17 @@ void sdsfree(sds s) {
     zfree(s-sizeof(struct sdshdr));
 }
 
+/* Remove sds header so we can use buffer as malloc'd byte array.
+ * Returns the contents of 's' usable as a full malloc'd C string. */
+char *sdsnative(sds s) {
+    if (!s) return NULL;
+
+    size_t len = sdslen(s);
+    char *base = s-sizeof(struct sdshdr);
+    memmove(base, s, len);
+    return zrealloc(base, len);
+}
+
 /* Set the sds string length to the length as obtained with strlen(), so
  * considering as content only up to the first null term character.
  *

src/sds.h

@@ -60,6 +60,7 @@ sds sdsempty(void);
 size_t sdslen(const sds s);
 sds sdsdup(const sds s);
 void sdsfree(sds s);
+char *sdsnative(sds s);
 size_t sdsavail(const sds s);
 sds sdsgrowzero(sds s, size_t len);
 sds sdscatlen(sds s, const void *t, size_t len);