[Backtracing] Fix a few niggles.

It turns out that .copyBytes() checks that the source doesn't provide more
bytes than the destination will accept.

rdar://117681625

Author: al45tair

stdlib/public/Backtracing/ArrayImageSource.swift

@@ -42,7 +42,7 @@ struct ArrayImageSource<T>: ImageSource {
         throw ArrayImageSourceError.outOfBoundsRead(addr, requested)
       }
 
-      buffer.copyBytes(from: $0[Int(addr)...])
+      buffer.copyBytes(from: $0[Int(addr)..<Int(addr+requested)])
     }
   }
 }

stdlib/public/Backtracing/CachingMemoryReader.swift

@@ -75,7 +75,7 @@ public class CachingMemoryReader<T: MemoryReader>: MemoryReader {
       let maxBytes = pageSize - offset
       let chunk = min(remaining, maxBytes)
 
-      buffer[done..<done+chunk].copyBytes(from: page[offset...])
+      buffer[done..<done+chunk].copyBytes(from: page[offset..<offset+chunk])
 
       offset = 0
       done += chunk

stdlib/public/Backtracing/MemoryReader.swift

@@ -245,8 +245,8 @@ extension MemoryReader {
         throw MemserverError(message: "Unreadable at \(hex(addr))")
       }
 
-      if done + Int(reply.len) > bytes.count {
-        throw MemserverError(message: "Overrun at \(hex(addr)) trying to read \(bytes.count) bytes")
+      if buffer.count - done < Int(reply.len) {
+        throw MemserverError(message: "Overrun at \(hex(addr)) trying to read \(buffer.count) bytes")
       }
 
       let ret = try safeRead(fd,