stb_truetype: fix bug in handling of stbtt_hheap that could cause severe badness

stb_truetype.h

@@ -43,7 +43,7 @@
 //       Hou Qiming                 Derek Vinyard
 //       Rob Loach                  Cort Stratton
 //       Kenney Phillis Jr.         github:oyvindjam
-//                                  github:vassvik
+//       Brian Costabile            github:vassvik
 //       
 // VERSION HISTORY
 //
@@ -2412,7 +2412,7 @@ static void *stbtt__hheap_alloc(stbtt__hheap *hh, size_t size, void *userdata)
          hh->num_remaining_in_head_chunk = count;
       }
       --hh->num_remaining_in_head_chunk;
-      return (char *) (hh->head) + size * hh->num_remaining_in_head_chunk;
+      return (char *) (hh->head) + sizeof(stbtt__hheap_chunk) + size * hh->num_remaining_in_head_chunk;
    }
 }