Fix VpNumOfChars calculation for the longest case (#366)

Author: tompng

ext/bigdecimal/bigdecimal.c

@@ -5028,7 +5028,7 @@ VpNumOfChars(Real *vp,const char *pszFmt)
       case 'E':
 	/* fall through */
       default:
-	nc = BASE_FIG*(vp->Prec + 2)+6; /* 3: sign + exponent chars */
+	nc = BASE_FIG * vp->Prec + 25; /* "-0."(3) + digits_chars + "e-"(2) + 64bit_exponent_chars(19) + null(1) */
     }
     return nc;
 }

test/bigdecimal/test_bigdecimal.rb

@@ -1503,6 +1503,9 @@ def test_inspect
     assert_equal("0.123456789012e0", BigDecimal("0.123456789012").inspect)
     assert_equal("0.123456789012e4", BigDecimal("1234.56789012").inspect)
     assert_equal("0.123456789012e-4", BigDecimal("0.0000123456789012").inspect)
+    s = '-0.123456789e-1000000000000000008'
+    x = BigDecimal(s)
+    assert_equal(s, x.inspect) unless x.infinite?
   end
 
   def test_power