Fix a possible NULL pointer dereference in following an OOM error

in sqlite3ExprIsInteger(). Problem found by OSS-Fuzz.

FossilOrigin-Name: 5ec655e8e817c1ed3bfb2e576745a7cef441494ad7baf1bf9f8895e98ac19c5a

src/expr.c

@@ -1860,6 +1860,7 @@ int sqlite3ExprContainsSubquery(Expr *p){
 */
 int sqlite3ExprIsInteger(Expr *p, int *pValue){
   int rc = 0;
+  if( p==0 ) return 0;  /* Can only happen following on OOM */
 
   /* If an expression is an integer literal that fits in a signed 32-bit
   ** integer, then the EP_IntValue flag will have already been set */