AK: Correct ByteBuffer::{overwrite,slice*} bounds check

Author: https://github.com/alimpfard
Commit: cf5941c972
Pull-request: https://github.com/SerenityOS/serenity/pull/1661
Reviewed-by: https://github.com/Dexesttp
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/itamar8910

AK/ByteBuffer.h

@@ -183,10 +183,10 @@ public:
     {
         if (is_null())
             return {};
-        if (offset >= this->size())
-            return {};
-        if (offset + size >= this->size())
-            size = this->size() - offset;
+
+        // I cannot hand you a slice I don't have
+        ASSERT(offset + size <= this->size());
+
         return wrap(offset_pointer(offset), size);
     }
 
@@ -194,10 +194,10 @@ public:
     {
         if (is_null())
             return {};
-        if (offset >= this->size())
-            return {};
-        if (offset + size >= this->size())
-            size = this->size() - offset;
+
+        // I cannot hand you a slice I don't have
+        ASSERT(offset + size <= this->size());
+
         return copy(offset_pointer(offset), size);
     }
 
@@ -222,7 +222,7 @@ public:
     void overwrite(size_t offset, const void* data, size_t data_size)
     {
         // make sure we're not told to write past the end
-        ASSERT(offset + data_size < size());
+        ASSERT(offset + data_size <= size());
         __builtin_memcpy(this->data() + offset, data, data_size);
     }