gpu/utils: fix null pointer arithmetic in mp_log_source()

strchr() can return NULL when no newline is found. next was computed
as end + 1 before the null check, which is undefined behaviour.

Moved next assignment after the null check.

Author: PatriciaBucataru

video/out/gpu/utils.c

@@ -339,9 +339,11 @@ void mp_log_source(struct mp_log *log, int lev, const char *src)
         return;
     while (*src) {
         const char *end = strchr(src, '\n');
-        const char *next = end + 1;
+        const char *next;
         if (!end)
             next = end = src + strlen(src);
+        else
+            next = end + 1;
         mp_msg(log, lev, "[%3d] %.*s\n", line, (int)(end - src), src);
         line++;
         src = next;