configure: Do not ignore malloc value

configure: Do not ignore malloc value

Not checking the value of malloc will cause a warning with GCC 10.1,
which may result in configuration failure, with the following line in
config.log:

config-temp/qemu-conf.c:2:18: error: ignoring return value of ‘malloc’ declared with attribute ‘warn_unused_result’ [-Werror=unused-result]
    2 | int main(void) { malloc(1); return 0; }
      |                  ^~~~~~~~~

Signed-off-by: Leonid Bloch <lb.workbox@gmail.com>
Message-Id: <20200524221204.9791-1-lb.workbox@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>