write: add return code checks when used

These were just warnings in test code but it still makes it nicer to not
generate them.

tests/libtest/lib556.c

@@ -84,9 +84,11 @@ int test(char *URL)
         sleep(1); /* avoid ctl-10 dump */
 #endif
 
-        if(iolen)
+        if(iolen) {
           /* send received stuff to stdout */
-          write(STDOUT_FILENO, buf, iolen);
+          if(!write(STDOUT_FILENO, buf, iolen))
+            break;
+        }
         total += iolen;
 
       } while(((res == CURLE_OK) || (res == CURLE_AGAIN)) && (total < 129));

tests/server/tftpd.c

@@ -605,7 +605,8 @@ static ssize_t write_behind(struct testcase *test, int convert)
     }
     /* formerly
        putc(c, file); */
-    write(test->ofile, &c, 1);
+    if(1 != write(test->ofile, &c, 1))
+      break;
     skipit:
     prevchar = c;
   }