Avoid passing NULL pointers to memcmp() or memcpy(), even when the

"number-of-bytes" argument is passed 0.

FossilOrigin-Name: 56ff72ab44288296efc99a608f7edc4346366a50

src/printf.c

@@ -841,7 +841,7 @@ void sqlite3StrAccumAppend(StrAccum *p, const char *z, int N){
   assert( p->accError==0 || p->nAlloc==0 );
   if( p->nChar+N >= p->nAlloc ){
     enlargeAndAppend(p,z,N);
-  }else{
+  }else if( N ){
     assert( p->zText );
     p->nChar += N;
     memcpy(&p->zText[p->nChar-N], z, N);