Make UBP nil/count checks debug-mode only

Every other Unsafe(Mutable)BufferPointer precondition in this file is debug-mode only.

This information is already part of the documentation for this function, and it's really hard to get rid of these branches and traps otherwise.

Author: karwa

stdlib/public/core/UnsafeBufferPointer.swift.gyb

@@ -401,9 +401,9 @@ extension Unsafe${Mutable}BufferPointer {
   public init(
     @_nonEphemeral start: Unsafe${Mutable}Pointer<Element>?, count: Int
   ) {
-    _precondition(
+    _debugPrecondition(
       count >= 0, "Unsafe${Mutable}BufferPointer with negative count")
-    _precondition(
+    _debugPrecondition(
       count == 0 || start != nil,
       "Unsafe${Mutable}BufferPointer has a nil start and nonzero count")
     _position = start