Buffer written into itself

In one fatal error code path, snprintf() is given its output buffer as
an input string.

src/jdk.jdwp.agent/share/native/libjdwp/debugInit.c line 645:

    (void)snprintf(buf, sizeof(buf), "JDWP %s", buf);

This was found by compiling with GCC 8.1


$ make all
Building target 'all' in configuration 'linux-x86_64-server-release'
Updating support/modules_libs/jdk.jdwp.agent/libjdwp.so due to 1 file(s)
/home/stooke/dev/openjdk/jdk12/src/jdk.jdwp.agent/share/native/libjdwp/debugInit.c: In function ‘jniFatalError’:
/home/stooke/dev/openjdk/jdk12/src/jdk.jdwp.agent/share/native/libjdwp/debugInit.c:650:24: error: passing argument 1 to restrict-qualified parameter aliases with argument 4 [-Werror=restrict]
         (void)snprintf(buf, sizeof(buf), "JDWP %s", buf);
                        ^~~ ~~~
/home/stooke/dev/openjdk/jdk12/src/jdk.jdwp.agent/share/native/libjdwp/debugInit.c: In function ‘jniFatalError.constprop’:
/home/stooke/dev/openjdk/jdk12/src/jdk.jdwp.agent/share/native/libjdwp/debugInit.c:650:48: error: ‘%s’ directive output may be truncated writing up to 511 bytes into a region of size 507 [-Werror=format-truncation=]
         (void)snprintf(buf, sizeof(buf), "JDWP %s", buf);
                                                ^~ ~~~
/home/stooke/dev/openjdk/jdk12/src/jdk.jdwp.agent/share/native/libjdwp/debugInit.c:650:15: note: ‘snprintf’ output between 6 and 517 bytes into a destination of size 512
         (void)snprintf(buf, sizeof(buf), "JDWP %s", buf);
               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
gmake[3]: *** [Lib-jdk.jdwp.agent.gmk:54: /home/stooke/dev/openjdk/jdk12/build/linux-x86_64-server-release/support/native/jdk.jdwp.agent/libjdwp/debugInit.o] Error 1
gmake[2]: *** [make/Main.gmk:215: jdk.jdwp.agent-libs] Error 2
gmake[2]: *** Waiting for unfinished jobs....
^Cmake[1]: *** [/home/stooke/dev/openjdk/jdk12/make/Init.gmk:310: main] Interrupt
make: *** [/home/stooke/dev/openjdk/jdk12/make/Init.gmk:186: all] Interrupt