Use fread() return value to count bytes

This commit changes the behavior of dcc6502, so that it consults the
return value of fread(), and then increments the value of byte_count
by the number returned by fread(), rather than implicitly incrementing
it every time.

Previously, a warning occurred during compilation, because the return value
of fread() was ignored. Instead, the number of bytes in the input file were
counted implicitly by incrementing a byte_count variable after every fread()
call.

Additionally, I created a two-byte test input file consisting of the
bytes #$a9ff, which corresponds to LDA #$FF. When dcc6502 tried to
disassemble this input file, it reported that the file had a size of three
bytes. It reported the first two opcodes correctly, but then incorrectly
displayed a BRK as the third opcode.

After this change, the input file now has a reported size of two bytes,
without the phantom BRK opcode at the end.

dcc6502.c

@@ -769,8 +769,8 @@ int main(int argc, char *argv[]) {
 
     byte_count = 0;
     while(!feof(input_file) && ((options.org + byte_count) <= 0xFFFFu) && (byte_count < options.max_num_bytes)) {
-        fread(&buffer[options.org + byte_count], 1, 1, input_file);
-        byte_count++;
+        size_t bytes_read = fread(&buffer[options.org + byte_count], 1, 1, input_file);
+        byte_count += bytes_read;
     }
 
     fclose(input_file);