examples/http2-upload: fix a crash

[rgeorgiev583]

If index.html does not exist in the directory from which the example
is invoked, the fopen(upload, "rb") invocation in setup would fail,
returning NULL. This value is subsequently passed as the FILE* argument
of the fread invocation in the read_callback function, which is the
actual cause of the crash (apparently fread assumes that argument to
be non-null).

In addition, mitigate some possible crashes of similar origin.

[bagder]

'make checksrc' found a few code style nits:

  RUN      checksrc
./http2-download.c:153:5: warning: if with space (SPACEBEFOREPAREN)
   if (!t->out)
     ^
./http2-download.c:154:3: warning: badly placed open brace (BRACEPOS)
   {
   ^
./http2-download.c:155:99: warning: Longer than 79 columns (LONGLINE)
     fprintf(stderr, "error: could not open file %s for writing: %s\n", filename, strerror(errno));
./http2-upload.c:182:5: warning: if with space (SPACEBEFOREPAREN)
   if (!out)
     ^
./http2-upload.c:183:3: warning: badly placed open brace (BRACEPOS)
   {
   ^
./http2-upload.c:184:97: warning: Longer than 79 columns (LONGLINE)
     fprintf(stderr, "error: could not open file %s for writing: %s\n", upload, strerror(errno));
./http2-upload.c:191:5: warning: if with space (SPACEBEFOREPAREN)
   if (stat(upload, &file_info))
     ^
./http2-upload.c:192:3: warning: badly placed open brace (BRACEPOS)
   {
   ^
./http2-upload.c:193:85: warning: Longer than 79 columns (LONGLINE)
     fprintf(stderr, "error: could not stat file %s: %s\n", upload, strerror(errno));
./http2-upload.c:200:5: warning: if with space (SPACEBEFOREPAREN)
   if (!i->in)
     ^
./http2-upload.c:201:3: warning: badly placed open brace (BRACEPOS)
   {
   ^
./http2-upload.c:202:97: warning: Longer than 79 columns (LONGLINE)
     fprintf(stderr, "error: could not open file %s for reading: %s\n", upload, strerror(errno));

[bagder]

ping @rgeorgiev583 ?

[rgeorgiev583]

Sorry for not responding; was a bit busy recently. Am also sorry for overlooking the code style guide. I'm pushing the fixes now; hope the checks would turn out fine after that :)

[bagder]

Thanks!