patch 8.0.1227: undefined left shift in readfile()

Problem:    Undefined left shift in readfile(). (Brian 'geeknik' Carpenter)
Solution:   Add cast to unsigned. (Dominique Pelle, closes #2253)
diff --git a/src/fileio.c b/src/fileio.c
index 5755156..82659be 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -1956,17 +1956,17 @@
 		    {
 			if (fio_flags & FIO_ENDIAN_L)
 			{
-			    u8c = (*--p << 24);
-			    u8c += (*--p << 16);
-			    u8c += (*--p << 8);
+			    u8c = (unsigned)*--p << 24;
+			    u8c += (unsigned)*--p << 16;
+			    u8c += (unsigned)*--p << 8;
 			    u8c += *--p;
 			}
 			else	/* big endian */
 			{
 			    u8c = *--p;
-			    u8c += (*--p << 8);
-			    u8c += (*--p << 16);
-			    u8c += (*--p << 24);
+			    u8c += (unsigned)*--p << 8;
+			    u8c += (unsigned)*--p << 16;
+			    u8c += (unsigned)*--p << 24;
 			}
 		    }
 		    else    /* UTF-8 */