set minimum capacity to one if it's zero in Segment constructor

Allocating an empty Segment causes problems for later computations, so
we avoid it even if the caller would otherwise permit it.

src/heap.cpp

@@ -346,6 +346,10 @@ class Segment {
     map(map)
   {
     if (desired) {
+      if (minimum == 0) {
+        minimum = 1;
+      }
+      
       assert(context, desired >= minimum);
 
       capacity_ = desired;