Abort on prematurate EOF when reading headers

src/fmail.c

@@ -236,6 +236,10 @@ read_headers(FILE *in, string_buffer_t *headers)
                 n = 0;
             }
         }
+        else if ( c == EOF ) {
+            /* EOF before reaching the end of headers, abort. */
+            errx(EXIT_FAILURE, "cannot read mail headers");
+        }
         else {
             sb_addc(headers, c);
             n += 1;