OGR/GMT: Error when parsing strings that contain the special `@` characters

[seisman]

Description of the problem

Original reported in GenericMappingTools/pygmt#2592 (comment).

pygmt-2m6nvbqu.gmt.txt

Attached above is an OGR/GMT file generated by the Fiona package. The first few lines of the file are:

# @VGMT1.0 @GPOLYGON
# @R-4506555.399/-3052902.89568/3804018.88695/5612191.52675               
# @Je3857
# @Jp"+proj=merc +a=6378137 +b=6378137 +lat_ts=0 +lon_0=0 +x_0=0 +y_0=0 +k=1 +units=m +nadgrids=@null +wktext +no_defs"
# @Jw"PROJCS[\"WGS 84 / Pseudo-Mercator\",GEOGCS[\"WGS 84\",DATUM[\"WGS_1984\",SPHEROID[\"WGS 84\",6378137,298.257223563,AUTHORITY[\"EPSG\",\"7030\"]],AUTHORITY[\"EPSG\",\"6326\"]],PRIMEM[\"Greenwich\",0,AUTHORITY[\"EPSG\",\"8901\"]],UNIT[\"degree\",0.0174532925199433,AUTHORITY[\"EPSG\",\"9122\"]],AUTHORITY[\"EPSG\",\"4326\"]],PROJECTION[\"Mercator_1SP\"],PARAMETER[\"central_meridian\",0],PARAMETER[\"scale_factor\",1],PARAMETER[\"false_easting\",0],PARAMETER[\"false_northing\",0],UNIT[\"metre\",1],AXIS[\"Easting\",EAST],AXIS[\"Northing\",NORTH],EXTENSION[\"PROJ4\",\"+proj=merc +a=6378137 +b=6378137 +lat_ts=0 +lon_0=0 +x_0=0 +y_0=0 +k=1 +units=m +nadgrids=@null +wktext +no_defs\"],AUTHORITY[\"EPSG\",\"3857\"]]"
# @NNPOINTS
# @Tstring
# FEATURE_DATA
>

Plotting the file (gmt plot pygmt-2m6nvbqu.gmt -pdf map) results in errors:

gmtinfo [ERROR]: Bad OGR/GMT: @n not allowed before FEATURE_DATA
plot [ERROR]: Bad OGR/GMT: @n not allowed before FEATURE_DATA

The error is because GMT incorrectly parse the @null string as an invalid ORG/GMT code (@n).

System information

• Operating system: Linux
• GMT version (gmt --version): 6.4.0