fix "src/dmierror.c:55:9: warning[-Wanalyzer-possible-null-argument]: use of possibly-NULL buf where non-null expected"

Fix the following error found by covscan,
    python-dmidecode-3.12.2/src/dmierror.c:55:9: warning[-Wanalyzer-possible-null-argument]: use of possibly-NULL buf where non-null expected
    /usr/include/python3.9/Python.h:30: included_from: Included from here.
    python-dmidecode-3.12.2/src/dmierror.c:32: included_from: Included from here.
    /usr/include/string.h:61:14: note: argument 1 of ���memset��� must be non-null
    #   53|           va_start(ap, fmt);
    #   54|           buf = (char *) malloc(4098);
    #   55|->         memset(buf, 0, 4098);
    #   56|
    #   57|           if( buf == NULL ) {

Author: coiby

src/dmierror.c

@@ -52,7 +52,6 @@ void _pyReturnError(void *exception, const char *fname, int line, const char *fm
 
         va_start(ap, fmt);
         buf = (char *) malloc(4098);
-        memset(buf, 0, 4098);
 
         if( buf == NULL ) {
                 // Backup routine if we can't get the needed memory
@@ -64,6 +63,7 @@ void _pyReturnError(void *exception, const char *fname, int line, const char *fm
                 return;
         }
 
+        memset(buf, 0, 4098);
         // Set the error state and message
         snprintf(buf, 4096, "[%s:%i] %s", fname, line, fmt);
         PyErr_Format(exception, buf, ap);