Fix off by one error in position quert, and crashes when using double enumerables

src/lib/Queries/Position.vala

@@ -23,7 +23,7 @@ namespace Invercargill {
                 return base_tracker.has_next();
             }
             public override PositionItemPair<T> get_next() {
-                return new PositionItemPair<T>(position++, base_tracker.get_next());
+                return new PositionItemPair<T>(++position, base_tracker.get_next());
             }
             
         }

src/lib/Safety.vala

@@ -112,10 +112,10 @@ namespace Invercargill {
     internal ulong safely_read_ulong_from_array(ulong[] array, int index) {
         return ((ulong[])array)[index];
     }
-    internal float safely_read_float_from_array(float[] array, int index) {
+    internal float safely_read_float_from_array(float?[] array, int index) {
         return ((float?[])array)[index];
     }
-    internal double safely_read_double_from_array(double[] array, int index) {
+    internal double? safely_read_double_from_array(double?[] array, int index) {
         return ((double?[])array)[index];
     }
 
@@ -146,10 +146,10 @@ namespace Invercargill {
     internal void safely_assign_ulong_to_array(ulong[] array, int index, ulong value) {
         ((ulong[])array)[index] = (ulong)value;
     }
-    internal void safely_assign_float_to_array(float[] array, int index, float value) {
+    internal void safely_assign_float_to_array(float?[] array, int index, float? value) {
         ((float?[])array)[index] = (float?)value;
     }
-    internal void safely_assign_double_to_array(double[] array, int index, double value) {
+    internal void safely_assign_double_to_array(double?[] array, int index, double? value) {
         ((double?[])array)[index] = (double?)value;
     }