rbindlist(): better initial allocation size

Also adjust the minimal hash table size to avoid shift overflow on
size=0 and free=0 on size=1.

Author: aitap

src/hash.c

@@ -15,7 +15,7 @@ struct hash_tab {
 static const double default_load_factor = .75;
 
 static R_INLINE size_t get_next_pow2(size_t n) {
-  if (n <= 1) return 1;
+  if (n <= 2) return 2;
   n--;
   n |= n >> 1;
   n |= n >> 2;

src/rbindlist.c

@@ -356,7 +356,7 @@ SEXP rbindlist(SEXP l, SEXP usenamesArg, SEXP fillArg, SEXP idcolArg, SEXP ignor
     int ansloc=0;
     if (factor) {
       char warnStr[1000] = "";
-      hashtab * marks = hash_create(1024);
+      hashtab * marks = hash_create(xlength(longestLevels));
       int nLevel=0, allocLevel=0;
       SEXP *levelsRaw = NULL;  // growing list of SEXP pointers. Raw since managed with raw realloc.
       if (orderedFactor) {