string append more protection

CMakeLists.txt

@@ -3,7 +3,7 @@ cmake_policy(SET CMP0017 NEW) # need include() with .cmake
 project(PIP)
 set(PIP_MAJOR    3)
 set(PIP_MINOR    19)
-set(PIP_REVISION 0)
+set(PIP_REVISION 1)
 set(PIP_SUFFIX   )
 set(PIP_COMPANY  SHS)
 set(PIP_DOMAIN   org.SHS)

libs/main/text/pistring.cpp

@@ -300,6 +300,7 @@ void PIString::appendFromChars(const char * c, int s, const char * codepage) {
 	if (s == 0) return;
 	int old_sz = size_s();
 	if (s == -1) s = strlen(c);
+	if (s <= 0) return;
 #ifdef PIP_ICU
 	UErrorCode e((UErrorCode)0);
 	UConverter * cc = ucnv_open(codepage, &e);