Fix some possible memory leaks and make some methods const.

This patch fixes a couple places where there were some memory
leaks on error and also makes some methods const that can be.
Found with cppcheck.

async.cc

@@ -23,12 +23,12 @@
 # include  "netlist.h"
 # include  <cassert>
 
-bool NetAssign::is_asynchronous()
+bool NetAssign::is_asynchronous() const
 {
       return true;
 }
 
-bool NetCondit::is_asynchronous()
+bool NetCondit::is_asynchronous() const
 {
       return false;
 }
@@ -82,7 +82,7 @@ bool NetProc::is_asynchronous()
       return false;
 }
 
-bool NetProcTop::is_asynchronous()
+bool NetProcTop::is_asynchronous() const
 {
       if (type_ == IVL_PR_INITIAL)
 	    return false;

lexor.lex

@@ -1288,6 +1288,7 @@ static void line_directive()
       if (cp == cpr) {
 	    VLerror(yylloc, "Invalid #line directive (missing space after "
 	                    "file name).");
+	    delete[] buf;
 	    return;
       }
       cp = cpr;
@@ -1296,6 +1297,7 @@ static void line_directive()
       unsigned long lineno = strtoul(cp, &cpr, 10);
       if (cp == cpr) {
 	    VLerror(yylloc, "Invalid line number for #line directive.");
+	    delete[] buf;
 	    return;
       }
       cp = cpr;
@@ -1305,6 +1307,7 @@ static void line_directive()
       if ((size_t)(cpr-yytext) != strlen(yytext)) {
 	    VLerror(yylloc, "Invalid #line directive (extra garbage after "
 	                    "line number).");
+	    delete[] buf;
 	    return;
       }
 

netlist.h

@@ -2317,7 +2317,7 @@ class NetAssign : public NetAssignBase {
       explicit NetAssign(NetAssign_*lv, NetExpr*rv);
       ~NetAssign();
 
-      bool is_asynchronous();
+      bool is_asynchronous() const;
 
       virtual bool emit_proc(struct target_t*) const;
       virtual int match_proc(struct proc_match_t*);
@@ -2496,7 +2496,7 @@ class NetCondit  : public NetProc {
       virtual NexusSet* nex_input(bool rem_out = true);
       virtual void nex_output(NexusSet&o);
 
-      bool is_asynchronous();
+      bool is_asynchronous() const;
       bool synth_async(Design*des, NetScope*scope,
 		       const NetBus&nex_map, NetBus&nex_out);
 
@@ -3162,7 +3162,7 @@ class NetProcTop  : public LineInfo, public Attrib {
       const NetScope*scope() const;
 
 	/* Return true if this process represents combinational logic. */
-      bool is_asynchronous();
+      bool is_asynchronous() const;
 
 	/* Create asynchronous logic from this thread and return true,
 	   or return false if that cannot be done. */