Fix std::out_of_range from string_view::find (#7)

The string_view::find operation was unexpectedly throwing an
out_of_range exception in cases where the position was nonzero
and the string wasn't found. 

This corrects those cases by accounting for the position, and adds
unit tests for validation.

Fixes #6

include/bpstd/detail/string_view.inl

@@ -293,7 +293,7 @@ namespace bpstd {
     }
 
     const auto offset = pos;
-    const auto increments = size() - v.size();
+    const auto increments = size() - v.size() - offset;
 
     for (auto i = 0u; i <= increments; ++i) {
       const auto j = i + offset;

single_include/bpstd/string_view.hpp

@@ -788,7 +788,7 @@ namespace bpstd {
     }
 
     const auto offset = pos;
-    const auto increments = size() - v.size();
+    const auto increments = size() - v.size() - offset;
 
     for (auto i = 0u; i <= increments; ++i) {
       const auto j = i + offset;

tests/bpstd/string_view.test.cpp

@@ -545,6 +545,23 @@ TEST_CASE("string_view::find", "[operations]")
         REQUIRE( result == 0u );
       }
     }
+    SECTION("Argument is string, offset in string, no match")
+    {
+        bpstd::string_view s1 = "01234567890ABCDEFGHIJ";
+        bpstd::string_view s2 = "01";
+
+        auto result = s1.find(s2);
+        SECTION("prefix find ok")
+        {
+            REQUIRE(result != bpstd::string_view::npos);
+            REQUIRE(s1.size() > 10);
+        }
+        auto result1 = s1.find(s2, 10);
+        SECTION("nonmatching substring")
+        {
+            REQUIRE(result1 == bpstd::string_view::npos);
+        }
+    }
   }
 }