w32_stack: convert buffer length param to `size_t`

In both `git_win32__stack_format` and `git_win32__stack`, we handle
buffer lengths via an integer variable. As we only ever pass buffer
sizes to it, this should be a `size_t` though to avoid loss of
precision. As we also use it to compare with other `size_t` variables,
this also silences signed/unsigned comparison warnings.

src/win32/w32_stack.c

@@ -76,7 +76,7 @@ int git_win32__stack_compare(
 }
 
 int git_win32__stack_format(
-	char *pbuf, int buf_len,
+	char *pbuf, size_t buf_len,
 	const git_win32__stack__raw_data *pdata,
 	const char *prefix, const char *suffix)
 {
@@ -171,7 +171,7 @@ int git_win32__stack_format(
 }
 
 int git_win32__stack(
-	char * pbuf, int buf_len,
+	char * pbuf, size_t buf_len,
 	int skip,
 	const char *prefix, const char *suffix)
 {

src/win32/w32_stack.h

@@ -116,7 +116,7 @@ int git_win32__stack_compare(
  * @param suffix String written after each frame; defaults to "\n".
  */
 int git_win32__stack_format(
-	char *pbuf, int buf_len,
+	char *pbuf, size_t buf_len,
 	const git_win32__stack__raw_data *pdata,
 	const char *prefix, const char *suffix);
 
@@ -132,7 +132,7 @@ int git_win32__stack_format(
  * @param suffix String written after each frame; defaults to "\n".
  */
 int git_win32__stack(
-	char * pbuf, int buf_len,
+	char * pbuf, size_t buf_len,
 	int skip,
 	const char *prefix, const char *suffix);