util/genvif: Fix array bounds check
util/genvif: Fix array bounds check

When removing the "-flto" flag when building genvif, gcc warns that that
the array can be out of bounds. This could occur if pushback_count was
set to a negative value since the if statement would be true:

util/genvif.c: In function ‘get_next_tag’:
util/genvif.c:1131:24: error: array subscript [-1, 2147483646] is
outside array bounds of ‘char[20]’ [-Werror=array-bounds]
 1131 |   return pushback_stack[--pushback_cnt];
      |          ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~
util/genvif.c:1116:6: note: while referencing ‘pushback_stack’
 1116 | char pushback_stack[20];
      |      ^~~~~~~~~~~~~~

BRANCH=none
BUG=b:172020503
TEST=none

Signed-off-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>
Change-Id: I1bc84f1b58ceb0b921c2d51bf94ac378211d556c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3229990
Reviewed-by: Daisuke Nojiri <fd5f93af191bf7e8f73ea71cc3c0f66b41b1dd49@chromium.org>
Reviewed-by: Eric Yilun Lin <2f2733c7206ea2b76bd2db1905cc5e8a75841c48@google.com>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec