Added assembler check for string outside Data Segment.

Programs/testPrograms/diagnostics/stringInProgram.asm

@@ -0,0 +1,19 @@
+; A string written into the Program Segment.
+;
+; This used to assemble. The string contributed no bytes and no message: the second pass
+; has a case for putting a string in the Data buffer and none for the Program buffer, so
+; it fell through the switch and was gone. A program written this way would branch to a
+; label that pointed at whatever followed instead.
+;
+; It is refused rather than emitted, because a string in Program Memory could not be read
+; by the program holding it. No instruction on this machine reads Program Memory; only the
+; memory controller can. Anybody writing this meant #Data.
+
+#Program
+
+start:
+  SETD.0 Greeting
+  HALT
+
+Greeting:
+"this is not where this goes"