Reduce # of time limit checks when in logical sig evaluation

While reviewing scan time performance, the time limit checks have a
notable effect on scan time. I tested reducing the number of time checks
to every n'th signature evaluation. For the given sample which has a
large number of very tiny embedded scans, I found that overall scan time
was best when checked every 10th signature. Reducing the number of
checks any further had no noticable effect.

Bear in mind that these numbers include daily/main/bytecode CVD load
time:

| current           | mod 2             | mod 10            |
| ----------------- | ----------------- | ----------------- |
| 63.848 s ±1.188 s | 61.773 s ±0.652 s | 59.831 s ±0.975 s |

| mod 50            | mod 100           | mod 1000          |
| ----------------- | ----------------- | ----------------- |
| 59.279 s ±1.652 s | 59.198 s ±1.147 s | 59.440 s ±1.304 s |

libclamav/matcher.c

@@ -959,6 +959,12 @@ static cl_error_t lsig_eval(cli_ctx *ctx, struct cli_matcher *root, struct cli_a
         if (CL_SUCCESS != status) {
             goto done;
         }
+
+        // Check time limit here, because bytecode functions may take a while.
+        status = cli_checktimelimit(ctx);
+        if (CL_SUCCESS != status) {
+            goto done;
+        }
     }
 
 done:
@@ -1018,10 +1024,15 @@ cl_error_t cli_exp_eval(cli_ctx *ctx, struct cli_matcher *root, struct cli_ac_da
             break;
         }
 
-        if (cli_checktimelimit(ctx) != CL_SUCCESS) {
-            cli_dbgmsg("Exceeded scan time limit while evaluating logical and yara signatures (max: %u)\n", ctx->engine->maxscantime);
-            status = CL_ETIMEOUT;
-            break;
+        if (i % 10 == 0) {
+            // Check the time limit every n'th lsig.
+            // In testing with a large signature set, we found n = 10 to be just as fast as 100 or
+            // 1000 and has a significant performance improvement over checking with every lsig.
+            status = cli_checktimelimit(ctx);
+            if (CL_SUCCESS != status) {
+                cli_dbgmsg("Exceeded scan time limit while evaluating logical and yara signatures (max: %u)\n", ctx->engine->maxscantime);
+                break;
+            }
         }
     }