Fix: ignoring return value of 'fread' declared with attribute 'warn_unused_result'

[yuce]

When I try to make the project using GCC 13.3.0, I get the following errors about ignoring the return result of fread in the load_file functions in mqjs.c and example.c:

gcc -Wall -g -MMD -Werror -D_GNU_SOURCE -fno-math-errno -fno-trapping-math -Os -c -o mqjs.o mqjs.c
mqjs.c: In function ‘load_file’:
mqjs.c:253:5: error: ignoring return value of ‘fread’ declared with attribute ‘warn_unused_result’ [-Werror=unused-result]
  253 |     fread(buf, 1, buf_len, f);
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
make: *** [Makefile:109: mqjs.o] Error 1

And

gcc -Wall -g -MMD -Werror -D_GNU_SOURCE -fno-math-errno -fno-trapping-math -Os -c -o example.o example.c
example.c: In function ‘load_file’:
example.c:244:5: error: ignoring return value of ‘fread’ declared with attribute ‘warn_unused_result’ [-Werror=unused-result]
  244 |     fread(buf, 1, buf_len, f);
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
make: *** [Makefile:109: example.o] Error 1

This PR fixes that by exiting if buf_len bytes are not read.

[lindenb]

better: fprintf(stderr,"Can't read %d bytes in %s\n", buf_len, filename );

and also in the other file.

[bellard]

just ignoring the warning is acceptable too.