Fix STRFMT stopping at NUL characters in format spec strings

src/asm/actions.cpp

@@ -555,7 +555,7 @@ std::string act_StringFormat(
 	std::string str;
 	size_t argIndex = 0;
 
-	for (size_t i = 0; spec[i] != '\0';) {
+	for (size_t i = 0; i < spec.length();) {
 		if (int c = spec[i]; c != '%') {
 			str += c;
 			++i;

test/asm/strfmt.asm

@@ -26,3 +26,7 @@ PRINTLN STRFMT("%d eol %", 1)
 PRINTLN STRFMT("invalid %w spec", 42)
 
 PRINTLN STRFMT("one=%d two=%d three=%d", 1)
+
+DEF NUL EQUS STRFMT("%s \0 %s", "goodbye", "world")
+ASSERT #NUL === "goodbye \0 world"
+PRINTLN #NUL