Fix off-by-one error in amount of characters in strip

Closes numpy#26898.

Author: lysnikolaou

numpy/_core/src/umath/string_buffer.h

@@ -1281,9 +1281,7 @@ string_lrstrip_chars(Buffer<enc> buf1, Buffer<enc> buf2, Buffer<enc> out, STRIPT
             }
             num_bytes -= traverse_buf.num_bytes_next_character();
             j--;
-            if (j > 0) {
-                traverse_buf--;
-            }
+            traverse_buf--;
         }
     }
 

numpy/_core/tests/test_strings.py

@@ -463,6 +463,7 @@ def test_endswith(self, a, suffix, start, end, out, dt):
         ("xyxzx", "x", "yxzx"),
         (["xyzzyhelloxyzzy", "hello"], ["xyz", "xyz"],
          ["helloxyzzy", "hello"]),
+        (["ba", "ac", "baa", "bba"], "b", ["a", "ac", "aa", "a"]),
     ])
     def test_lstrip(self, a, chars, out, dt):
         a = np.array(a, dtype=dt)
@@ -488,6 +489,7 @@ def test_lstrip(self, a, chars, out, dt):
         ("xyxzx", "x", "xyxz"),
         (["xyzzyhelloxyzzy", "hello"], ["xyz", "xyz"],
          ["xyzzyhello", "hello"]),
+        (["ab", "ac", "aab", "abb"], "b", ["a", "ac", "aa", "a"]),
     ])
     def test_rstrip(self, a, chars, out, dt):
         a = np.array(a, dtype=dt)
@@ -511,6 +513,7 @@ def test_rstrip(self, a, chars, out, dt):
         ("xyxzx", "x", "yxz"),
         (["xyzzyhelloxyzzy", "hello"], ["xyz", "xyz"],
          ["hello", "hello"]),
+        (["bab", "ac", "baab", "bbabb"], "b", ["a", "ac", "aa", "a"]),
     ])
     def test_strip(self, a, chars, out, dt):
         a = np.array(a, dtype=dt)