Skip special case in mpc_pow_int() for special numbers

Closes #472

mpmath/libmp/libmpc.py

@@ -260,7 +260,7 @@ def mpc_pow_int(z, n, prec, rnd=round_fast):
     de = aexp - bexp
     abs_de = abs(de)
     exact_size = n*(abs_de + max(abc, bbc))
-    if exact_size < 10000:
+    if exact_size < 10000 and min(abc, bbc) >= 0:
         if de > 0:
             aman <<= de
             aexp = bexp

mpmath/tests/test_gammazeta.py

@@ -694,3 +694,7 @@ def test_issue_471():
     assert bernpoly(4, inf) == inf
     assert bernpoly(4, mpc(inf, 0)) == mpc(inf, 0)
     assert isnan(bernpoly(4, nan))
+
+def test_issue_472():
+    assert bernpoly(4, mpc(inf, 1e-50)) == mpc(inf, 0)
+    assert mpc(inf, 2)**4 == mpc(inf, 0)