Remove extra entry in error stack on macro not defined (#394)

While working on #392, I noticed that the macro-@ test (as well
as the line-continuation test, but for that one see #393)
printed an additional '@(-1)' entry which doesn't make sense.

src/asm/fstack.c

@@ -318,7 +318,7 @@ uint32_t fstk_RunMacro(char *s)
 {
 	struct sSymbol *sym = sym_FindMacro(s);
 
-	if (sym == NULL)
+	if (sym == NULL || sym->pMacro == NULL)
 		return 0;
 
 	pushcontext();
@@ -328,9 +328,6 @@ uint32_t fstk_RunMacro(char *s)
 	nCurrentStatus = STAT_isMacro;
 	strcpy(tzCurrentFileName, s);
 
-	if (sym->pMacro == NULL)
-		return 0;
-
 	pCurrentMacro = sym;
 	CurrentFlexHandle = yy_scan_bytes(pCurrentMacro->pMacro,
 					  strlen(pCurrentMacro->pMacro));

test/asm/macro-@.out

@@ -1,2 +1,2 @@
-ERROR: macro-@.asm(1) -> @(-1):
+ERROR: macro-@.asm(1):
     Macro '@' not defined

test/asm/macro-@.out.pipe

@@ -1,2 +1,2 @@
-ERROR: -(1) -> @(-1):
+ERROR: -(1):
     Macro '@' not defined