Fix bug on bitset_resize function in case of increase of size:

the first limb may not be cleared properly in some cases.
(Reported by David M. Lary)

m-bitset.h

@@ -268,8 +268,8 @@ m_bitset_resize (m_bitset_t v, size_t size)
     }
   } else if (size > old_size) {
     // Resize up the bitset: set to 0 new bits.
-    const size_t old_offset = old_size / M_B1TSET_LIMB_BIT;
-    for(size_t i = old_offset+1 ; i < offset; i++) {
+    const size_t old_offset = (old_size + M_B1TSET_LIMB_BIT - 1)/ M_B1TSET_LIMB_BIT;
+    for(size_t i = old_offset ; i < offset; i++) {
       v->ptr[i] = 0;
     }
     if (M_LIKELY(index != 0)) {