gpu/utils: fix null pointer arithmetic UB in mp_log_source

strchr() can return NULL when no newline is found. Computing end + 1
before the null check on end is undefined behaviour per C11 6.5.6.
Move the next assignment inside the else branch to avoid the UB.

video/out/gpu/utils.c

@@ -339,9 +339,12 @@ 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;
-        if (!end)
+        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;