Use unsigned integers in SDS header.

This raises the max string to 4GB without any downside.

src/sds.c

@@ -200,7 +200,10 @@ size_t sdsAllocSize(sds s) {
 void sdsIncrLen(sds s, int incr) {
     struct sdshdr *sh = (void*) (s-(sizeof(struct sdshdr)));
 
-    assert(sh->free >= incr);
+    if (incr >= 0)
+        assert(sh->free >= (unsigned int)incr);
+    else
+        assert(sh->len >= (unsigned int)(-incr));
     sh->len += incr;
     sh->free -= incr;
     assert(sh->free >= 0);
@@ -458,7 +461,7 @@ sds sdscatfmt(sds s, char const *fmt, ...) {
     i = initlen; /* Position of the next byte to write to dest str. */
     while(*f) {
         char next, *str;
-        int l;
+        unsigned int l;
         long long num;
         unsigned long long unum;
 

src/sds.h

@@ -39,8 +39,8 @@
 typedef char *sds;
 
 struct sdshdr {
-    int len;
-    int free;
+    unsigned int len;
+    unsigned int free;
     char buf[];
 };