rtlib/dos: write NUL character when DISABLE_WCHAR is used

-  ensure NUL terminating character is written on optimized
   conversion from ascii to wstring in the event that the
   copy is truncated - for example copying from a larger
   temporary string to a fixed length wstring and
   DISABLE_WCHAR is in use (previously HOST_DOS)

Conflicts:
- changelog.txt, resolved by adding changelog message to 1.10.3 section

Author: jayrm

changelog.txt

@@ -11,6 +11,7 @@ Version 1.10.3
 [fixed]
 - github #33: Bi include error in ntdef.bi 
 - bad code generation of operator cast() UDT ptr passed to byval UDT ptr parameter when matching constructors() are also present
+- rtlib/dos: ensure NUL terminating character is written on optimized conversion from ascii to wstring in the event that the copy is truncated - for example copying from a larger temporary string to a fixed length wstring and DISABLE_WCHAR is in use (previously HOST_DOS)
 
 
 Version 1.10.2

src/rtlib/strw_convfrom_str.c

@@ -37,10 +37,13 @@ ssize_t fb_wstr_ConvFromA(FB_WCHAR *dst, ssize_t dst_chars, const char *src)
 
 #if defined HOST_DOS
 	ssize_t chars = strlen(src);
-	if (chars > dst_chars)
+	if (chars > dst_chars) {
 		chars = dst_chars;
-
+	}
 	memcpy(dst, src, chars + 1);
+
+	/* ensure that the null terminator is written, string may have been truncated */
+	dst[chars] = '\0';
 	return chars;
 #else
 	/* plus the null-term (note: "n" in chars, not bytes!) */