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>

configure

@@ -4587,7 +4587,13 @@ fi
 if test "$tcmalloc" = "yes" ; then
   cat > $TMPC << EOF
 #include <stdlib.h>
-int main(void) { malloc(1); return 0; }
+int main(void) {
+    void *tmp = malloc(1);
+    if (tmp != NULL) {
+        return 0;
+    }
+    return 1;
+}
 EOF
 
   if compile_prog "" "-ltcmalloc" ; then
@@ -4603,7 +4609,13 @@ fi
 if test "$jemalloc" = "yes" ; then
   cat > $TMPC << EOF
 #include <stdlib.h>
-int main(void) { malloc(1); return 0; }
+int main(void) {
+    void *tmp = malloc(1);
+    if (tmp != NULL) {
+        return 0;
+    }
+    return 1;
+}
 EOF
 
   if compile_prog "" "-ljemalloc" ; then
@@ -6164,7 +6176,9 @@ if test "$sanitizers" = "yes" ; then
 #include <stdlib.h>
 int main(void) {
     void *tmp = malloc(10);
-    return *(int *)(tmp + 2);
+    if (tmp != NULL) {
+        return *(int *)(tmp + 2);
+    }
 }
 EOF
   if compile_prog "$CPU_CFLAGS -Werror -fsanitize=undefined" ""; then