patch 8.1.1384: using "int" for alloc() often results in compiler warnings

Problem:    Using "int" for alloc() often results in compiler warnings.
Solution:   Use "size_t" and remove type casts.  Remove alloc_check(), Vim
            only works with 32 bit ints anyway.
diff --git a/src/debugger.c b/src/debugger.c
index 78e32d3..b79f613 100644
--- a/src/debugger.c
+++ b/src/debugger.c
@@ -873,7 +873,7 @@
     // Replace K_SNR in function name with "<SNR>".
     if (!file && fname[0] == K_SPECIAL)
     {
-	name = alloc((unsigned)STRLEN(fname) + 3);
+	name = alloc(STRLEN(fname) + 3);
 	if (name == NULL)
 	    name = fname;
 	else