Issue #2949 Run background checks during long multiplications

py/mpz.c

@@ -444,8 +444,11 @@ STATIC size_t mpn_mul(mpz_dig_t *idig, mpz_dig_t *jdig, size_t jlen, mpz_dig_t *
         }
 
       ilen = id - oidig;
+      // check to prevent usb starvation
+      RUN_BACKGROUND_TASKS;
     }
     
+
     return ilen;
 }