Fixed scanning a negative number as an unsigned value

e.g. sscanf("-1", "%zu", &v)

Thanks to @sezero for the test case

src/stdlib/SDL_string.c

@@ -105,6 +105,10 @@ SDL_ScanUnsignedLong(const char *text, int count, int radix, unsigned long *valu
     const char *textstart = text;
     unsigned long value = 0;
 
+    if (*text == '-') {
+        return SDL_ScanLong(text, count, radix, (long *)valuep);
+    }
+
     if (radix == 16 && SDL_strncmp(text, "0x", 2) == 0) {
         text += 2;
     }
@@ -218,6 +222,10 @@ SDL_ScanUnsignedLongLong(const char *text, int count, int radix, Uint64 * valuep
     const char *textstart = text;
     Uint64 value = 0;
 
+    if (*text == '-') {
+        return SDL_ScanLongLong(text, count, radix, (Sint64 *)valuep);
+    }
+
     if (radix == 16 && SDL_strncmp(text, "0x", 2) == 0) {
         text += 2;
     }