Improve strictness of max-scantime

Some files that take longer to scan may exceed the scan time by a fair
bit during the actual pattern matching part of the scan (vs.
decompressing archives or running bytecode functions, or whatever).

ClamAV does pattern matching and hashing of files in chunks.
This change adds a time check before each chunk is matched.

Author: val-ms

libclamav/matcher.c

@@ -1254,6 +1254,12 @@ cl_error_t cli_scan_fmap(cli_ctx *ctx, cli_file_t ftype, bool filetype_only, str
     }
 
     while (offset < ctx->fmap->len) {
+        if (cli_checktimelimit(ctx) != CL_SUCCESS) {
+            cli_dbgmsg("Exceeded scan time limit while scanning fmap (max: %u)\n", ctx->engine->maxscantime);
+            ret = CL_ETIMEOUT;
+            goto done;
+        }
+
         bytes = MIN(ctx->fmap->len - offset, SCANBUFF);
         if (!(buff = fmap_need_off_once(ctx->fmap, offset, bytes)))
             break;