mpi_mul_hlp: microoptimization

If c == 0, no need to add it to *d.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

library/bignum.c

@@ -1607,10 +1607,10 @@ void mpi_mul_hlp( size_t i,
 
     t++;
 
-    do {
+    while( c != 0 )
+    {
         *d += c; c = ( *d < c ); d++;
     }
-    while( c != 0 );
 }
 
 /*