Don't append the stale static string in fmsgattr_str()

When this function was called multiple times (e.g. multiple messages in a pkt),
it would keep appending to the current (static) string, eventually overflowing
the buffer and corrupting the stack. Although reported in pktdump, it could
have also happened with fmsgdump if passed multiple stored message (.msg) files.

Fixes #295 reported by Nelgin

src/sbbs3/fmsgdump.c

@@ -32,6 +32,7 @@ const char* fmsgattr_str(uint16_t attr)
 {
 	char str[64] = "";
 
+	str[0] = '\0';
 #define FIDO_ATTR_CHECK(a, f) if(a&FIDO_##f)	sprintf(str + strlen(str), "%s%s", str[0] == 0 ? "" : ", ", #f);
 	FIDO_ATTR_CHECK(attr, PRIVATE);
 	FIDO_ATTR_CHECK(attr, CRASH);

src/sbbs3/pktdump.c

@@ -52,6 +52,7 @@ const char* fmsgattr_str(uint16_t attr)
 {
 	static char str[64] = "";
 
+	str[0] = '\0';
 #define FIDO_ATTR_CHECK(a, f) if(a&FIDO_##f)	sprintf(str + strlen(str), "%s%s", str[0] == 0 ? "" : ", ", #f);
 	FIDO_ATTR_CHECK(attr, PRIVATE);
 	FIDO_ATTR_CHECK(attr, CRASH);