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.

test/asm/assert-nul.asm

@@ -0,0 +1 @@
+assert x, "oops \0 null"

test/asm/assert-nul.err

@@ -0,0 +1,2 @@
+FATAL: Assertion messages cannot contain '\0' characters
+    at assert-nul.asm(1)

test/asm/section-name-nul.asm

@@ -0,0 +1,2 @@
+SECTION "test\0 foo", ROM0
+SECTION "test\0 bar", ROM0

test/asm/section-name-nul.err

@@ -0,0 +1,2 @@
+FATAL: Section names cannot contain '\0' characters
+    at section-name-nul.asm(1)