Disallow NUL characters in section names and assertion messages

Allowing these in object files would lose anything after the '\0'
when RGBLINK reads the object.

src/asm/output.cpp

@@ -162,6 +162,10 @@ void out_CreatePatch(uint32_t type, Expression const &expr, uint32_t ofs, uint32
 void out_CreateAssert(
     AssertionType type, Expression const &expr, std::string const &message, uint32_t ofs
 ) {
+	if (message.find('\0') != std::string::npos) {
+		fatal("Assertion messages cannot contain '\\0' characters");
+	}
+
 	Assertion &assertion = assertions.emplace_front();
 
 	initPatch(assertion.patch, type, expr, ofs);

src/asm/section.cpp

@@ -544,6 +544,10 @@ void sect_NewSection(
     SectionSpec const &attrs,
     SectionModifier mod
 ) {
+	if (name.find('\0') != std::string::npos) {
+		fatal("Section names cannot contain '\\0' characters");
+	}
+
 	for (SectionStackEntry &entry : sectionStack) {
 		if (entry.section && entry.section->name == name) {
 			fatal("Section \"%s\" is already on the stack", name.c_str());