libgfortran: Fix compilation of gf_vsnprintf

The fallback function (gf_vsnprintf) to provide a vsnprintf function
if the system library doesn't have one would not compile due to the
variable name for the string's destination buffer not being updated
after the refactor in 2018 in edaaef60.

This updates the internal logic of gf_vsnprintf to now use the str
variable defined in the function signature.

libgfortran/ChangeLog:

2024-04-04  Ian McInerney  <i.mcinerney17@imperial.ac.uk>

	* runtime/error.c (gf_vsnprintf): Fix compilation

Signed-off-by: Ian McInerney <i.mcinerney17@imperial.ac.uk>

libgfortran/runtime/error.c

@@ -142,15 +142,15 @@ gf_vsnprintf (char *str, size_t size, const char *format, va_list ap)
 {
   int written;
 
-  written = vsprintf(buffer, format, ap);
+  written = vsprintf(str, format, ap);
 
   if (written >= size - 1)
     {
-      /* The error message was longer than our buffer.  Ouch.  Because
+      /* The error message was longer than the string size.  Ouch.  Because
 	 we may have messed up things badly, report the error and
 	 quit.  */
-#define ERROR_MESSAGE "Internal error: buffer overrun in gf_vsnprintf()\n"
-      write (STDERR_FILENO, buffer, size - 1);
+#define ERROR_MESSAGE "Internal error: string overrun in gf_vsnprintf()\n"
+      write (STDERR_FILENO, str, size - 1);
       write (STDERR_FILENO, ERROR_MESSAGE, strlen (ERROR_MESSAGE));
       sys_abort ();
 #undef ERROR_MESSAGE