patch 8.2.1892: valgrind warns for using uninitialized access in tests

Problem:    Valgrind warns for using uninitialized access in tests.
Solution:   Fix condition for breaking out of loop. (Dominique Pellé,
            closes #7187)
diff --git a/src/terminal.c b/src/terminal.c
index 867a0e3..049e2e7 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -4718,6 +4718,7 @@
 	    {
 		int c = cell.chars[i];
 		int pc = prev_cell.chars[i];
+		int should_break = c == NUL || pc == NUL;
 
 		// For the first character NUL is the same as space.
 		if (i == 0)
@@ -4727,7 +4728,7 @@
 		}
 		if (c != pc)
 		    same_chars = FALSE;
-		if (c == NUL || pc == NUL)
+		if (should_break)
 		    break;
 	    }
 	    same_attr = vtermAttr2hl(cell.attrs)