Assembler: @ command errors

[markarmb]

Bug encountered by many students who use @-1 in thier Fill.asm programs.

Numbers outside the range [0, 32767] cause undetected errors in assembly.
This affects all instances of the assembler as used in Assembler, CPUEmulator and HardwareSimulator.

////////////////////////////////////////////////////////////////////////
// FILE:  bad_a.asm
//
// No error messages are generated by this file.

// source      binary
@0          // 0000000000000000     OK
@-1         // 1111111111111111     illegal c-instuction generated
@-2         // 1111111111111110     illegal c-instuction generated
@32767      // 0111111111111111     OK
@32768      // 0000000000010000     RAM address for variable "32768"
@32769      // 0000000000010001     RAM address for variable "32769"
@foo        // 0000000000010010     RAM address for variable "foo"
@32770      // 0000000000001011     ROM address for label "32770"
@-32768     // 1000000000000000     illegal c-instuction generated
@-32769     // 0000000000010011     RAM address for variable "-32769"
@-32770     // 0000000000001100     ROM address for label "-32770"
(32770)     //                      label "32770" (ROM addr = 11)
@0          // 0000000000000000     OK
(-32770)    //                      label "-32770" (ROM addr = 12)

// There are at least two bugs here:
//   o  numbers outside the range [-32678, 32767] are being treated
//      as symbols in all contexts.  (Parser bug?)
//   o  negative numbers are not disallowed in '@' commands.