Redo 71afa3e0 and 71afa3e0 so powten(max_leg_digits) is 1e18

This is what it was before 71afa3e0.  This is the largest power of
ten that fits in an int64.

libgnucash/engine/gnc-numeric.cpp

@@ -42,7 +42,7 @@
 
 static QofLogModule log_module = "qof";
 
-static const uint8_t max_leg_digits{17};
+static const uint8_t max_leg_digits{18};
 static const int64_t pten[] = { 1, 10, 100, 1000, 10000, 100000, 1000000,
                                10000000, 100000000, 1000000000,
                                INT64_C(10000000000), INT64_C(100000000000),
@@ -50,7 +50,8 @@ static const int64_t pten[] = { 1, 10, 100, 1000, 10000, 100000, 1000000,
                                INT64_C(100000000000000),
                                INT64_C(1000000000000000),
                                INT64_C(10000000000000000),
-                               INT64_C(100000000000000000)};
+                               INT64_C(100000000000000000),
+                               INT64_C(1000000000000000000)};
 #define POWTEN_OVERFLOW -5
 
 int64_t
@@ -79,7 +80,7 @@ GncNumeric::GncNumeric(GncRational rr)
 
 GncNumeric::GncNumeric(double d) : m_num(0), m_den(1)
 {
-    static uint64_t max_leg_value{INT64_C(100000000000000000)};
+    static uint64_t max_leg_value{INT64_C(1000000000000000000)};
     if (std::isnan(d) || fabs(d) > max_leg_value)
     {
         std::ostringstream msg;

libgnucash/engine/test/gtest-gnc-numeric.cpp

@@ -185,12 +185,12 @@ TEST(gncnumeric_constructors, test_string_constructor)
     GncNumeric neg_decimal_frac_nozero("-.12345");
     EXPECT_EQ(-12345, neg_decimal_frac_nozero.num());
     EXPECT_EQ(100000, neg_decimal_frac_nozero.denom());
-    GncNumeric big_denom(".12345678901234567");
-    EXPECT_EQ(12345678901234567, big_denom.num());
-    EXPECT_EQ(100000000000000000, big_denom.denom());
-    GncNumeric too_big_denom(".123456789012345678");
-    EXPECT_EQ(12345678901234567, too_big_denom.num());
-    EXPECT_EQ(100000000000000000, too_big_denom.denom());
+    GncNumeric big_denom(".123456789012345678");
+    EXPECT_EQ(123456789012345678, big_denom.num());
+    EXPECT_EQ(1000000000000000000, big_denom.denom());
+    GncNumeric too_big_denom(".1234567890123456789");
+    EXPECT_EQ(123456789012345678, too_big_denom.num());
+    EXPECT_EQ(1000000000000000000, too_big_denom.denom());
 }
 
 TEST(gncnumeric_output, string_output)