Fix a heap-buffer-overflow in in str strip! methods.

This issue was originally discovered by OSS-Fuzz:

https://issues.oss-fuzz.com/issues/428404023

The root cause was that str_strip_bang modified the string content and
length in-place but failed to null-terminate the string at its new
length.

When this modified, non-null-terminated string was duplicated, the
buffer may be resized, dropping the old null terminator (via str_uminus
-> mrb_str_dup -> str_replace -> str_share). When this is later passed
to mrb_raisef using the %!s format specifier, mrb_vformat called strlen
on the underlying non-null terminated buffer pointer.

The fix adds explicit null-termination in str_strip_bang,
str_lstrip_bang, and str_rstrip_bang after the string length is updated.

mrbgems/mruby-string-ext/src/string.c

@@ -1513,10 +1513,12 @@ str_lstrip_bang(mrb_state *mrb, mrb_value self)
   if (start < len) {
     memmove(ptr, ptr + start, len - start);
     RSTR_SET_LEN(s, len - start);
+    ptr[len - start] = '\0';
   }
   else {
     /* All whitespace - make empty */
     RSTR_SET_LEN(s, 0);
+    ptr[0] = '\0';
   }
 
   return self;
@@ -1554,6 +1556,7 @@ str_rstrip_bang(mrb_state *mrb, mrb_value self)
 
   /* Truncate string */
   RSTR_SET_LEN(s, end);
+  ptr[end] = '\0';
 
   return self;
 }
@@ -1608,6 +1611,7 @@ str_strip_bang(mrb_state *mrb, mrb_value self)
 
   /* Set new length */
   RSTR_SET_LEN(s, end - start);
+  ptr[end - start] = '\0';
 
   return self;
 }