scanner: when bracelevel is negative, treat as zero.

This really should never happen, but this at least fixes the
breakage on Verilator.
diff --git a/src/scan.l b/src/scan.l
index 8400cf6..abe47f4 100644
--- a/src/scan.l
+++ b/src/scan.l
@@ -940,16 +940,16 @@
         "'"             ACTION_ECHO; BEGIN(CHARACTER_CONSTANT);
 	\"		ACTION_ECHO; BEGIN(ACTION_STRING);
 	{NL} {
-			++linenum;
-			ACTION_ECHO;
-			if (bracelevel == 0) {
-				if ( doing_rule_action )
-                    add_action( "\tYY_BREAK]""]\n" );
+                ++linenum;
+                ACTION_ECHO;
+                if (bracelevel <= 0) {
+                   if ( doing_rule_action )
+                      add_action( "\tYY_BREAK]""]\n" );
 
-                doing_rule_action = false;
-                BEGIN(SECT2);
-            }
-        }
+                   doing_rule_action = false;
+                   BEGIN(SECT2);
+                }
+             }
         .      ACTION_ECHO;
 }