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/diff.c b/src/diff.c
index 38ae0b4..3c72e12 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -537,7 +537,7 @@
 {
     diff_T	*dnew;
 
-    dnew = (diff_T *)alloc((unsigned)sizeof(diff_T));
+    dnew = (diff_T *)alloc(sizeof(diff_T));
     if (dnew != NULL)
     {
 	dnew->df_next = dp;
@@ -1123,7 +1123,7 @@
     {
 	len = STRLEN(tmp_orig) + STRLEN(tmp_new)
 				      + STRLEN(tmp_diff) + STRLEN(p_srr) + 27;
-	cmd = alloc((unsigned)len);
+	cmd = alloc(len);
 	if (cmd == NULL)
 	    return FAIL;
 
@@ -1218,7 +1218,7 @@
     if (esc_name == NULL)
 	goto theend;
     buflen = STRLEN(tmp_orig) + STRLEN(esc_name) + STRLEN(tmp_new) + 16;
-    buf = alloc((unsigned)buflen);
+    buf = alloc(buflen);
     if (buf == NULL)
 	goto theend;