check return value of fgets

check return value of fgets

Following warning was observed:
 init.c: In function 'mutt_init':
 mutt.h:53:15: warning: ignoring return value of 'fgets_unlocked', declared with attribute warn_unused_result [-Wunused-result]
  #define fgets fgets_unlocked

 init.c:4053:7: note: in expansion of macro 'fgets'
        fgets(buffer, sizeof(buffer), f);
        ^~~~~
Added code that checks return value.

Signed-off-by: Stefan Assmann <sassmann@kpanic.de>