stb_ds: Fix addn when n=0

In this case, the array pointer may still be 0 even after the
maybegrow.

Fixes issue #1015.

stb_ds.h

@@ -544,8 +544,8 @@ extern void * stbds_shmode_func(size_t elemsize, int mode);
 #define stbds_arrpush          stbds_arrput  // synonym
 #define stbds_arrpop(a)        (stbds_header(a)->length--, (a)[stbds_header(a)->length])
 #define stbds_arraddn(a,n)     ((void)(stbds_arraddnindex(a, n)))    // deprecated, use one of the following instead:
-#define stbds_arraddnptr(a,n)  (stbds_arrmaybegrow(a,n), stbds_header(a)->length += (n), &(a)[stbds_header(a)->length-(n)])
-#define stbds_arraddnindex(a,n)(stbds_arrmaybegrow(a,n), stbds_header(a)->length += (n), stbds_header(a)->length-(n))
+#define stbds_arraddnptr(a,n)  (stbds_arrmaybegrow(a,n), (n) ? (stbds_header(a)->length += (n), &(a)[stbds_header(a)->length-(n)]) : (a))
+#define stbds_arraddnindex(a,n)(stbds_arrmaybegrow(a,n), (n) ? (stbds_header(a)->length += (n), stbds_header(a)->length-(n)) : stbds_arrlen(a))
 #define stbds_arraddnoff       stbds_arraddnindex
 #define stbds_arrlast(a)       ((a)[stbds_header(a)->length-1])
 #define stbds_arrfree(a)       ((void) ((a) ? STBDS_FREE(NULL,stbds_header(a)) : (void)0), (a)=NULL)