MDEV-30613 output_core_info crashes in my_read()

and my_getwd(). The cause is my_errno define which
depends on my_thread_var being a not null pointer
otherwise it will be de-referenced and cause
a SEGV already in the signal handler.

Replace uses of these functions in the output_core_info
using posix read/getcwd functions instead.

The getwd fallback in my_getcwd isn't needed as
its been obsolute for a very long time.

Thanks Vladislav Vaintroub for diagnosis and posix
recommendation.

sql/signal_handler.cc

@@ -27,6 +27,7 @@
 
 #ifdef __WIN__
 #include <crtdbg.h>
+#include <direct.h>
 #define SIGNAL_FMT "exception 0x%x"
 #else
 #define SIGNAL_FMT "signal %d"
@@ -67,27 +68,27 @@ static inline void output_core_info()
     my_safe_printf_stderr("Writing a core file...\nWorking directory at %.*s\n",
                           (int) len, buff);
   }
-  if ((fd= my_open("/proc/self/limits", O_RDONLY, MYF(0))) >= 0)
+  if ((fd= open("/proc/self/limits", O_RDONLY, MYF(0))) >= 0)
   {
     my_safe_printf_stderr("Resource Limits:\n");
-    while ((len= my_read(fd, (uchar*)buff, sizeof(buff),  MYF(0))) > 0)
+    while ((len= read(fd, (uchar*)buff, sizeof(buff))) > 0)
     {
       my_write_stderr(buff, len);
     }
-    my_close(fd, MYF(0));
+    close(fd);
   }
 #ifdef __linux__
-  if ((fd= my_open("/proc/sys/kernel/core_pattern", O_RDONLY, MYF(0))) >= 0)
+  if ((fd= open("/proc/sys/kernel/core_pattern", O_RDONLY, MYF(0))) >= 0)
   {
-    len= my_read(fd, (uchar*)buff, sizeof(buff),  MYF(0));
+    len= read(fd, (uchar*)buff, sizeof(buff));
     my_safe_printf_stderr("Core pattern: %.*s\n", (int) len, buff);
-    my_close(fd, MYF(0));
+    close(fd);
   }
-  if ((fd= my_open("/proc/version", O_RDONLY, MYF(0))) >= 0)
+  if ((fd= open("/proc/version", O_RDONLY)) >= 0)
   {
-    len= my_read(fd, (uchar*)buff, sizeof(buff),  MYF(0));
+    len= read(fd, (uchar*)buff, sizeof(buff));
     my_safe_printf_stderr("Kernel version: %.*s\n", (int) len, buff);
-    my_close(fd, MYF(0));
+    close(fd);
   }
 #endif
 #elif defined(__APPLE__) || defined(__FreeBSD__)
@@ -101,11 +102,14 @@ static inline void output_core_info()
   {
     my_safe_printf_stderr("Kernel version: %.*s\n", (int) len, buff);
   }
-#else
+#elif defined(HAVE_GETCWD)
   char buff[80];
-  my_getwd(buff, sizeof(buff), 0);
-  my_safe_printf_stderr("Writing a core file at %s\n", buff);
-  fflush(stderr);
+
+  if (getcwd(buff, sizeof(buff)))
+  {
+    my_safe_printf_stderr("Writing a core file at %.*s\n", (int) sizeof(buff), buff);
+    fflush(stderr);
+  }
 #endif
 }