Extract a class function bits() to return a size value.

Helps to pre-determine overflow. Also correct the original implementation
in operator *=, which got the wrong answer.

src/libqof/qof/qofint128.cpp

@@ -163,6 +163,16 @@ QofInt128::abs() const noexcept
     return *this;
 }
 
+uint
+QofInt128::bits() const noexcept
+{
+    uint bits {static_cast<uint>(m_hi == 0 ? 0 : 64)};
+    uint64_t temp {(m_hi == 0 ? m_lo : m_hi)};
+    for (;temp > 0; temp >>= 1)
+        ++bits;
+    return bits;
+}
+
 
 QofInt128
 QofInt128::operator-() const noexcept
@@ -305,23 +315,8 @@ QofInt128::operator*= (const QofInt128& b) noexcept
         m_flags |= overflow;
         return *this;
     }
-    uint abits {}, bbits {};
-    uint64_t temp {m_lo};
-    do
-        ++abits;
-    while (temp >>= 1);
-    temp = m_hi;
-    do
-        ++abits;
-    while (temp >>= 1);
-    temp = b.m_lo;
-    do
-        ++bbits;
-    while (temp >>= 1);
-    temp = b.m_hi;
-    do
-        ++bbits;
-    while (temp >>= 1);
+
+    uint abits {bits()}, bbits {b.bits()};
     if (abits + bbits > maxbits)
     {
         m_flags |= overflow;

src/libqof/qof/qofint128.hpp

@@ -179,6 +179,11 @@ enum // Values for m_flags
  */
     bool isZero() const noexcept;
 
+/**
+ * @return the number of bits used to represent the value
+ */
+    uint bits() const noexcept;
+
 /**
  * Fills a supplied buffer with a representation of the number in base 10. If
  * the QofInt128 is overflowed or NaN it will contain the words "Overflow" or