Bug#20768820 MAIN.BIGINT TEST FAILS WHEN BUILT WITH GCC 5 IN RELEASE BUILD

Problem: with gcc5 in optmized mode, (- LLONG_MIN ) yields integer overflow.
Fix: In ull2dec() change the loop which counts the number of decimal_digit_t's
(cherry picked from commit b37d8bcc24f82f8e15c5f6e2243c8937af74acb7)

Author: Tor Didriksen

strings/decimal.c

@@ -1064,26 +1064,34 @@ int double2decimal(double from, decimal_t *to)
 
 static int ull2dec(ulonglong from, decimal_t *to)
 {
-  int intg1, error=E_DEC_OK;
-  ulonglong x=from;
+  int intg1;
+  int error= E_DEC_OK;
+  ulonglong x= from;
   dec1 *buf;
 
   sanity(to);
 
-  for (intg1=1; from >= DIG_BASE; intg1++, from/=DIG_BASE) ;
+  if (from == 0)
+    intg1= 1;
+  else
+  {
+    /* Count the number of decimal_digit_t's we need. */
+    for (intg1= 0; from != 0; intg1++, from/= DIG_BASE)
+      ;
+  }
   if (unlikely(intg1 > to->len))
   {
-    intg1=to->len;
-    error=E_DEC_OVERFLOW;
+    intg1= to->len;
+    error= E_DEC_OVERFLOW;
   }
-  to->frac=0;
-  to->intg=intg1*DIG_PER_DEC1;
+  to->frac= 0;
+  to->intg= intg1 * DIG_PER_DEC1;
 
-  for (buf=to->buf+intg1; intg1; intg1--)
+  for (buf= to->buf + intg1; intg1; intg1--)
   {
-    ulonglong y=x/DIG_BASE;
-    *--buf=(dec1)(x-y*DIG_BASE);
-    x=y;
+    ulonglong y= x / DIG_BASE;
+    *--buf=(dec1)(x - y * DIG_BASE);
+    x= y;
   }
   return error;
 }