Fix file pointer management in FileStream. Fixes rejectedsoftware/vibe.d#1684.

source/vibe/core/file.d

@@ -464,6 +464,7 @@ struct FileStream {
 			cb => eventDriver.files.read(m_fd, ctx.ptr, dst, mode, cb),
 			cb => eventDriver.files.cancelRead(m_fd)
 		);
+		ctx.ptr += res[2];
 		enforce(res[1] == IOStatus.ok, "Failed to read data from disk.");
 		return res[2];
 	}

tests/vibe.core.file.d

@@ -12,16 +12,23 @@ void main()
 {
 	auto f = openFile("test.dat", FileMode.createTrunc);
 	assert(f.size == 0);
+	assert(f.tell == 0);
 	f.write(bytes!(1, 2, 3, 4, 5));
 	assert(f.size == 5);
+	assert(f.tell == 5);
 	f.seek(0);
+	assert(f.tell == 0);
 	f.write(bytes!(1, 2, 3, 4, 5));
 	assert(f.size == 5);
+	assert(f.tell == 5);
 	f.write(bytes!(6, 7, 8, 9, 10));
 	assert(f.size == 10);
+	assert(f.tell == 10);
 	ubyte[5] dst;
 	f.seek(2);
+	assert(f.tell == 2);
 	f.read(dst);
+	assert(f.tell == 7);
 	assert(dst[] == bytes!(3, 4, 5, 6, 7));
 	f.close();