os: fix os.read_file and os.read_bytes for 0 sized /proc/ files on Linux (fix #15852) (#15853)

vlib/strings/builder.c.v

@@ -16,6 +16,17 @@ pub fn new_builder(initial_size int) Builder {
 	return res
 }
 
+// reuse_as_plain_u8_array allows using the Builder instance as a plain []u8 return value.
+// It is useful, when you have accumulated data in the builder, that you want to
+// pass/access as []u8 later, without copying or freeing the buffer.
+// NB: you *should NOT use* the string builder instance after calling this method.
+// Use only the return value after calling this method.
+[unsafe]
+pub fn (mut b Builder) reuse_as_plain_u8_array() []u8 {
+	unsafe { b.flags.clear(.noslices) }
+	return *b
+}
+
 // write_ptr writes `len` bytes provided byteptr to the accumulated buffer
 [unsafe]
 pub fn (mut b Builder) write_ptr(ptr &u8, len int) {