Fix adb get-serialno parsing

The function assumed that the raw output of "adb get-serialno" was a
NUL-terminated string, but it is not the case.

It this output did not end with a space or a new line character, then
sc_str_truncate() would write '\0' over the last character. Even worse,
if the output was empty, then sc_str_truncate() would write
out-of-bounds.

Avoid the error-prone sc_str_truncate() util function.

app/src/adb/adb.c

@@ -417,7 +417,7 @@ sc_adb_get_serialno(struct sc_intr *intr, unsigned flags) {
     }
 
     char buf[128];
-    ssize_t r = sc_pipe_read_all_intr(intr, pid, pout, buf, sizeof(buf));
+    ssize_t r = sc_pipe_read_all_intr(intr, pid, pout, buf, sizeof(buf) - 1);
     sc_pipe_close(pout);
 
     bool ok = process_check_success_intr(intr, pid, "adb get-serialno", flags);
@@ -429,7 +429,10 @@ sc_adb_get_serialno(struct sc_intr *intr, unsigned flags) {
         return NULL;
     }
 
-    sc_str_truncate(buf, r, " \r\n");
+    assert((size_t) r < sizeof(buf));
+    buf[r] = '\0';
+    size_t len = strcspn(buf, " \r\n");
+    buf[len] = '\0';
 
     return strdup(buf);
 }