Basic mid now throws an error on out-of-bounds parameters rather than returning

an uninitialised pointer (and crashing).

Fixes: #54

lang/basic/lib/lib.h

@@ -0,0 +1,9 @@
+#ifndef LIB_H
+#define LIB_H
+
+extern void error(int index);
+extern char* salloc(unsigned length);
+extern void sfree(char* c);
+
+#endif
+

lang/basic/lib/string.c

@@ -1,6 +1,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include "bc_string.h"
+#include "lib.h"
 
 /* $Id$ */
 
@@ -11,8 +12,6 @@
 	if (X == 0) \
 		return (0);
 
-extern char* salloc();
-
 int _length(String* str)
 {
 	okr(str);
@@ -158,7 +157,7 @@ String* _mid(int i1, int i2, String* s)
 
 	/*	printf("mid fcn called %d %d %s\n",i1,i2,s->strval);*/
 	if (i2 < 0 || i1 < -1)
-		return (s2); /* or error? */
+		error(3);
 	if (i1 == -1)
 		i1 = s->strlength;
 	s2 = _newstr(s->strval);