LTO build fails on warning message

The uninitialized variable found that caused the fail on build,
due to LTO builds with flag to check warnings as errors. Set the
variable to 0 initial value. The found issue was:

[100%] Built target xrow.test
src/box/sql/vdbe.c: In function ‘sqlVdbeExec’:
src/box/sql/vdbe.c:3691:11: error: ‘id’ may be used uninitialized
	in this function [-Werror=maybe-uninitialized]
  uint64_t id;
           ^
lto1: all warnings being treated as errors
lto-wrapper: fatal error: /usr/bin/c++ returned 1 exit status
compilation terminated.
/usr/bin/ld: error: lto-wrapper failed
collect2: error: ld returned 1 exit status

Closes #4378