use charAt() because length() is number of chars

Author: fluentfuture

mug-errorprone/src/main/java/com/google/mu/errorprone/Placeholder.java

@@ -53,10 +53,10 @@ public String toString() {
     // Find the full line surrounding the placeholder.
     int snippetStart = match.index();
     int snippetEnd = match.index() + match.length();
-    while (snippetStart > 0 && context.codePointAt(snippetStart - 1) != '\n') {
+    while (snippetStart > 0 && context.charAt(snippetStart - 1) != '\n') {
       snippetStart--;
     }
-    while (snippetEnd < context.length() && context.codePointAt(snippetEnd) != '\n') {
+    while (snippetEnd < context.length() && context.charAt(snippetEnd) != '\n') {
       snippetEnd++;
     }