SVG: Don't render missing glyphs

This bug is similar to the canvas bug of mozilla#6721.
I found this bug when I tried to run pdf2svg on a SVG file, and the generated
SVG could not be viewed in Chrome due to a SVG/XML parsing error:
"PCDATA invalid Char value 3"

Reduced test case:
- https://github.com/mozilla/pdf.js/files/1229507/pcdatainvalidchar.pdf
- expected: "hardware performance"
- Actual SVG source: "hardware\x03performance"
  (where "\x03" is a non-printable character, and invalid XML).

In terms of rendering, this bug is similar to mozilla#6721, where an unexpected glyph
appeared in the canvas renderer. This was fixed by mozilla#7023, which skips over
missing glyphs. This commit follows a similar logic.

The test case from mozilla#6721 can be used here too:
- https://github.com/mozilla/pdf.js/files/52205/issue6721_reduced.pdf
  expected: "Issue   6721"
  actual (before this patch): "Issue ààà6721"

src/display/svg.js

@@ -742,15 +742,23 @@ SVGGraphics = (function SVGGraphicsClosure() {
           x += -glyph * fontSize * 0.001;
           continue;
         }
-        current.xcoords.push(current.x + x * textHScale);
 
         var width = glyph.width;
         var character = glyph.fontChar;
         var spacing = (glyph.isSpace ? wordSpacing : 0) + charSpacing;
         var charWidth = width * widthAdvanceScale + spacing * fontDirection;
-        x += charWidth;
 
+        if (!glyph.isInFont && !font.missingFile) {
+          x += charWidth;
+          // TODO: To assist with text selection, we should replace the missing
+          // character with a space character if charWidth is not zero.
+          // But we cannot just do "character = ' '", because the ' ' character
+          // might actually map to a different glyph.
+          continue;
+        }
+        current.xcoords.push(current.x + x * textHScale);
         current.tspan.textContent += character;
+        x += charWidth;
       }
       if (vertical) {
         current.y -= x * textHScale;