Fix get_matrix() aliasing border rows when border > 1

[gaoflow]

What

[[False] * width] * self.border creates border references to the same inner list object. When border > 1, all top border rows (and all bottom border rows) in the returned matrix share a single backing list, so mutating any one of them silently corrupts the rest.

Reproducer

from qrcode.main import QRCode

qr = QRCode(border=4)
qr.add_data("1")
m = qr.get_matrix()
print(m[0] is m[1])   # True  (same object — bug!)
m[0][0] = True
print(m[1][0])         # True  (mutation propagated — bug!)

Fix

Replace the list-multiplication with list comprehensions so that every border row has its own independent list:

-        code = [[False] * width] * self.border
+        code = [[False] * width for _ in range(self.border)]
         ...
-        code += [[False] * width] * self.border
+        code += [[False] * width for _ in range(self.border)]

Tests

Added test_get_matrix_border_rows_not_aliased which verifies that, with border=4, every pair of top/bottom border rows is a distinct object and all values are False.

All 33 tests pass.