imagedev/floppy.cpp: fix out-of-bounds access to the seek sample (#12452)

Currently, when `floppy_sound_device::step` selects a new seek sample, the
value of `m_seek_samplepos` remains the same as it was the last time a seek
sample was played. This might be an invalid position for the new sample,
since the seek samples have different lengths. When this is the case, the next
call to `floppy_sound_device::sound_stream_update` makes an out-of-bounds memory
access as it reads the sample data.

Fix this by resetting `m_seek_samplepos` to 0 when a new seek sample is
selected.

This also makes the seek sounds more consistent, since previously they would
start from a semi-random position each time.

src/devices/imagedev/floppy.cpp

@@ -1639,6 +1639,9 @@ void floppy_sound_device::step(int zone)
 						}
 					}
 				}
+
+				// Start the new seek sound from the beginning.
+				m_seek_samplepos = 0;
 			}
 
 			// Changing the pitch does not always sound convincing