Trac #29249: Fixing determinant for the zero matrix over the p-adics

This fixes the bug below:
{{{
sage: matrix(Qp(37),[0]).determinant()
------------------------------------------------------------------------
---
UnboundLocalError                         Traceback (most recent call
last)
<ipython-input-323-f9ac4bb66315> in <module>()
----> 1 matrix(Zp(Integer(37)),[Integer(0)]).determinant()

/Applications/sage-dev/local/lib/python3.7/site-
packages/sage/matrix/matrix2.pyx in
sage.matrix.matrix2.Matrix.determinant
(build/cythonized/sage/matrix/matrix2.c:14548)()
   1640         R = self._base_ring
   1641         if hasattr(R, '_matrix_determinant'):
-> 1642             d = R._matrix_determinant(self)
   1643             self.cache('det', d)
   1644             return d

/Applications/sage-dev/local/lib/python3.7/site-
packages/sage/rings/padics/local_generic.py in _matrix_determinant(self,
M)
   1587                 break
   1588             val = curval
-> 1589             if S[pivi,pivj] == 0:
   1590                 if track_precision:
   1591                     return R(0, valdet + (n-piv)*val - shift)

UnboundLocalError: local variable 'pivi' referenced before assignment

}}}

URL: https://trac.sagemath.org/29249
Reported by: edgarcosta
Ticket author(s): Edgar Costa
Reviewer(s): Travis Scrimshaw

src/sage/rings/padics/local_generic.py

@@ -1554,6 +1554,10 @@ def _matrix_determinant(self, M):
             True
             sage: A.change_ring(QQ).det() == A.det()
             True
+            sage: matrix(Qp(37),[0]).determinant()
+            0
+            sage: matrix(Qp(37),[O(37)]).determinant()
+            O(37)
         """
         n = M.nrows()
 
@@ -1573,7 +1577,8 @@ def _matrix_determinant(self, M):
         valdet = 0
         val = -Infinity
         for piv in range(n):
-            curval = Infinity
+            pivi = pivj = piv
+            curval = S[pivi, pivj].valuation()
             for i in range(piv,n):
                 for j in range(piv,n):
                     v = S[i,j].valuation()